In order to have a unique religious system, it’s important to have many nuances such as festivals, holidays, rituals and more that are ‘exclusive’ to it. This will be an attempt to construct a Gaulish holiday with a Belgic nuance to it. While there is little to no documentation on holidays during the time of Gallia Belgica, there is no reason we can’t attempt or assume a hypothetical continuum of sorts between periods of time (IE Pre Christian Gaulish holiday to Pre Christian Roman Holiday to Pre Christian Germanic Holiday to Christian Holiday), even if it wasn’t the case in actuality. Provided we’re honest about the reality of the situation (readily admitting that we will never truly know), we can still make traditions that provide religious and cultural significance to Senobessus Bolgon, old Custom of the Belgae.
The name that I have chosen for this festival is Giamouretîmâ (Giamos: Winter and Uoretet: to help, release, relief). This is a compound word that was created by Segomâros Widugeni and myself, but since no other attested words exist Gallia Belgica pertaining to this idea of a festival, I find it appropriate.
Indo European peoples had various holiday commonalities. One such commonality is the Carnival or festival dedicated to bringing summer from winter, to drive out winter spirits so spring can come. The word carnival could’ve been of late Latin origin, which is another reason why we eschew it for the compound word Giamouretîmâ. Carnivals in Europe often had the idea of eating well one last time during this period due to the winter food storage running out and also an overturning of societal norms as well as overturning of sexual norms (for fertility purposes).
Some survivals in the Belgic regions include the idea of fat tuesday, carnival in Luxembourg, and Carnival of Binche. We should also take note of the Careto tradition in Portugal, even though it is not in the Belgic region. The reason we should note it is because the tradition itself is said to be a Prehistoric Celtic religious ritual, and if this is the case, we can use it as a comparison with the general template of Carnival and possibly extrapolate similarities between the Celtic, Germanic, and Roman for the purposes of our hypothetical (or even just invented) continuum. (Note: the only resource the author could find on this was an archived article, so this should be acknowledged as tenuous)
To find out which deity would fit into this festival, we could look into the traditions of the Germanic people that Tacitus spoke of.
“[The Reudigni, Aviones, Anglii, Varini, Eudoses, Saurines, and Nuitones] share a common worship of Nerthus, or Mother Earth. They believe that she takes part in human affairs, riding in a chariot among her people. On an island of the sea stands an inviolate grove, in which, veiled with a cloth, is a chariot that none but the priest may touch. The priest can feel the presence of the goddess in this holy of holies, and attends her with the deepest reverence as her chariot is drawn along by cows. Then follow days of rejoicing and merrymaking in every place that she condescends to visit and sojourn in. No one goes to war, no one takes up arms; every iron object is locked away. Then, and then only, are peace and quiet known and welcomed, until the goddess, when she has had enough of the society of men, is restored to her sacred precinct by the priest. After that, the chariot, the vestments, and (believe it if you will) the goddess herself, are cleansed in a secluded lake. This service is performed by slaves who are immediately afterwards drowned in the lake. Thus mystery begets terror and a pious reluctance to ask what that sight can be which is seen only by men doomed to die.” 
At first glance, Eponâ is a good choice considering her iconography of being shown in a cart in some places, and being associated with horses. Horses are important in this as they are associated with being psychopomps and therefore death, and winter is also associated with death. It’s a tenuous connection, but a connection nonetheless. However, her associations with fecundity wouldn’t disqualify Her from being fit into the holiday/festival. In fact, it could be argued that by Her alleged role as winter bringer allows Her to leave the land and give spring a chance to bloom.
There are other choices for gods as well, such as Boudina. Tacitus mentions that the Terra Mater figure was pulled by cattle instead of horses and first glance of Boudina would give the reader rationale to place Her in place of this Terra Mater. Linguistically Boudina is hard to fit in, since Her name is more likely connected to victory instead of it having to connotations of a bovine nature.
Another choice would be Nehalennia. Nehalennia is obscure as to what she was called upon for other than protection of travelers and seafaring. Her iconography however suggests that she was a god of fertility and horticulture as well, but there are a few other pieces of iconography that possibly point to a psychopomp function. Another connection to that function may include a myth of a blessed isle (recorded by Procopius by the time of the Salian Franks) found in a region where she had dedications, which could be a continued folk belief from the previous religions or a belief that came with the Franks. Within the region of Domburg alone, there are 28 inscriptions which show that in the region She was very important. If She was in fact a psychopomp, it could be reasoned that She had some dominion over the seasons (winter into summer). From here on out, Nehlennia will be the god that will be focused on for the festival due to her being a link between the Celtic cultures and Germanic cultures of Gallia Belgica and Germania Inferior, as well as her importance of trade and fertility and horticulture.
Moving on to the actual meat of the festival, we will look at Carnival as a continued survival of some pagan traditions.
First we have the Carnival of Binche which like the Careto tradition in Entrudo as mentioned above has costumed participants called Gilles causing a ruckus, albeit for a different purpose. The Gilles wave sticks to chase away the winter or evil spirits whilst throwing oranges at the passerby as a gift of the oncoming spring. We could guess this is a survival of older customs due to the phrase “Whoever in February by a variety of less honorable acts tries to drive out winter is not a Christian, but a pagan” being in alleged circulation at the same time of the synod of Leptines speaking out against excess in the month of February (Note: This resource is about the synod speaking out and not the alleged phrase). However, we should note the mention of the month February doesn’t necessarily mean that the original date would’ve been in that time of year, especially since Pope Gregory moved the date of the carnival and the fasting earlier.
The next holidays that are relevant to our continuum is found in Luxembourg, called Fuesend and Burgbrennen. Fuesend is observed on Candlemas Day through Ash Wednesday. The end of the carnival is marked with burning of the “Stréimännchen” (straw man”) on Ash Wednesday, symbolizing a happy death of winter (This is also similar to a custom in Slavic countries where the goddess Morena was burnt as an effigy during this period).
We don’t have to categorize the burning of an effigy as killing, but a releasing of the spirit or god. Burgbrennen also symbolizes the end of winter and rebirth of spring by lighting of ‘Beurgen’ (torches) on hilltops. This would take place around the time of the Spring Equinox .If we see the above statement of feasting due to winter storage ending being true, we could utilize the idea and place the start of the festival at Shrove Tuesday, placing it in the seventh week before Easter, and the day before Ash Wednesday. That might not be as ideal or practical than say putting the date on the Spring (Vernal) Equinox or the first full moon after the Spring Equinox (the later option would essentially give us Easter as the date). Another option would be to utilize both the concept of Shrove Tuesday and Easter dates, in which the participant feasts, which would lead to a religiously motivated fast (symbolizing when the ancestors food storage was up) and then the celebration of winter finally being released, giving way to spring.
It would then look like:
1. A public carnival like celebration involving masks, costumes, sticks and drums. The items aforementioned would be used to chase off winter spirits to ensure Spring and Summer eventually come. A feast would happen (communally or privately) that would involve meat, lard, and butter (though this can be amended for the vegetarian. An observation alone of how the ancestors would run out of meat during this time would suffice).
2. A religious fast would happen starting after the feast. Rich foods might be prudent to fast from, such as dairy, fat, meat and sugar.
3. On the day of the Equinox or full moon after the Equinox, an effigy of Nehalennia is burnt and sent to a body of water to release Her so she may bring spring and summer.
However, Pope Gregory the Great mandated that fasting would start on Ash Wednesday. Carnival was set before fasting to make sure there was a demarcation between the Pagan and Christian custom. If whatever indigenous festivals were indeed moved, to make it more authentic, we could move it to after the feast/fasting to be in line with our hypothetical continuum.
Thus, as a public/communal celebration, it would look like:
1. A religious fast would happen starting after the feast. Rich foods might be prudent to fast from, such as dairy, fat, meat and sugar. Period may vary for practicality reasons.
2. On the day of the Equinox or full moon after the Equinox, a public carnival like celebration involving masks, costumes, sticks and drums. The items aforementioned would be used to chase off winter spirits to ensure Spring and Summer eventually come. A feast would happen (communally or privately) that would involve meat, lard, and butter (though this can be amended for the vegetarian. An observation alone of how the ancestors would run out of meat during this time would suffice).
3. An effigy of Nehalennia is burnt to release Her to bring Spring.
Saint Eligius gives us one more idea for practicality purposes during this festival:
“I denounce and contest, that you shall observe no sacrilegious pagan customs. For no cause or infirmity should you consult magicians, diviners, sorcerers or incantators. ..Do not observe auguries… No influence attaches to the first work of the day or the [phase of the] moon. … [Do not] make vetulas, little deer or iotticos or set tables [for the house-elf] at night or exchange New Year gifts or supply superfluous drinks [a Yule midsummer custom]…No Christian. .. performs solestitia or dancing or leaping or diabolical chants. No Christian should presume to invoke the name of a demon, not Neptune or Orcus or Diana or Minerva or Geniscus… No one should observe Jove’s day in idleness. … No Christian should make or render any devotion to the gods of the trivium, where three roads meet, to the fanes or the rocks, or springs or groves or corners. None should presume to hang any phylacteries from the neck of man nor beast. ..None should presume to make lustrations or incantations with herbs, or to pass cattle through a hollow tree or ditch … No woman should presume to hang amber from her neck or call upon Minerva or other ill-starred beings in their weaving or dyeing. .. None should call the sun or moon lord or swear by them. .. No one should tell fate or fortune or horoscopes by them as those do who believe that a person must be what he was born to be.” 
A vetula is a type of corn dolly that is found in folk traditions in Indo European scope. The previously mentioned Russian tradition of Morena also mentions the drowning of their effigy after burning it, so they could no longer touch it and bring misfortune. James George Frazer comments:
“In the neighbourhood of Danzig the person who cuts the last ears of corn makes them into a doll, which is called the Corn-mother or the Old Woman and is brought home on the last waggon. In some parts of Holstein the last sheaf is dressed in women’s clothes and called the Corn-mother. It is carried home on the last waggon, and then thoroughly drenched with water. The drenching with water is doubtless a rain-charm. In the district of Bruck in Styria the last sheaf, called the Corn-mother, is made up into the shape of a woman by the oldest married woman in the village, of an age from 50 to 55 years. The finest ears are plucked out of it and made into a wreath, which, twined with flowers, is carried on her head by the prettiest girl of the village to the farmer or squire, while the Corn-mother is laid down in the barn to keep off the mice. In other villages of the same district the Corn-mother, at the close of harvest, is carried by two lads at the top of a pole. They march behind the girl who wears the wreath to the squire’s house, and while he receives the wreath and hangs it up in the hall, the Corn-mother is placed on the top of a pile of wood, where she is the centre of the harvest supper and dance.”
There is one more thing we do to flavor this festival if one chooses to honor Nehalennia during this festival. The loaves of bread from Nehalennia were identified as duivekater, which is a sacrificial loaf of bread in the shape of a shin bone.
This may have taken the place of animal sacrifice at some point way later in time. But as stated in the ritual format, plant offerings (processed and unprocessed) were a common sacrifice among the Belgic peoples, making this an appropriate idea to incorporate into our continuum. This duivekater could be shared among one’s household for hearth cult around the time of the festival and offered to the god dwelling in the corn dolly.
This can add more to the hearth practice of Senobessus Bolgon as well as fill out the options of the people who can’t have a communal or elaborate celebration of this festival while maintain it’s overall integrity. Should a person not be able to participate in the communal version of the festival (or would like to do both communal and hearth), the hearth version would look as such:
1. A corn dolly from the ‘last harvest’ would be created, and dwell in the home until the festival comes.
2. A religious fast would happen starting after the feast. Rich foods might be prudent to fast from, such as dairy, fat, meat and sugar. Period may vary for practicality reasons (1-28 days from the vernal equinox would be a realistic range). Offerings would be made to the corn dolly (Nehalennia in this case) for a good (and relatively short) winter as well as continued survival.
3. A duivekater (sacrificial bread) is made and shared within the home and with the corn dolly effigy.
4. On the day of the Equinox or full moon after the Equinox, the corn dolly effigy of Nehalennia is burnt and/or then placed in the nearest body of water to release Her so she may bring spring.
5. The fast would end. Another feast would happen and a bonfire would be made where stories would be told.
1. “Wat is carnaval?” | Fen Vlaanderen. Fenvlaanderen.be.
2. Careto’s Tradition https://web.archive.org/web/20100529040327/http://www.azibo.org/eng/caretosorigemeng.html 2006 by José Paulo Carvalho Pereira
3. Germania by Corneilius Tacitus 40. In The Agricola and Germania. Translated by Harold Mattingly. p. 134-135.
4. The Apple Branch: a Path to Celtic Ritual by Alexei Kondratiev p. 125-128
5. Celtic Religions in The Roman Period: Personal, Local, and Global edited by Ralph Haeussler & Anthony King 2018 (entry: Celtic Goddesses From Gallia Belgica and The Germaniae: Characteristics, Dedicants and Ritual Practices by Audrey Ferlut) P. 370
6. “Nehalenniatempel – Romeins verleden herleeft in Zeeland”. http://www.nehalennia-tempel.nl.
7. Celtic Religions in The Roman Period: Personal, Local, and Global edited by Ralph Haeussler & Anthony King 2018 (entry: Celtic Goddesses From Gallia Belgica and The Germaniae: Characteristics, Dedicants and Ritual Practices by Audrey Ferlut) P. 370
8. De Bellis by Procopius c. 540s CE book VIII translator: H. B. Dewin. 1916
9. The Origins of Old Germanic Studies in the Low Countries by Cornelis Dekke p. 209
10. Belgium & Luxembourg. Lonely Planet by Leanne Logan and Geert Cole 2007
11. “Wat is carnaval?” | Fen Vlaanderen. Fenvlaanderen.be.
12. “Geschiedenis van het carnavalsfeest”
16. Vita S. Eligius translated by Jo Ann McNamara Book II https://sourcebooks.fordham.edu/basis/eligius.asp
17. The Golden Bough by James George Frazer 1890, chapter 45
18. Roles of the Northern Goddess by Hilda Ellis Davidson 1998 P. 134