So remember when I said that I would answer ten questions about the InCryptid universe? Well, I'm still taking questions, but here's your fifth answer!
"I'm not asking for calendar of holidays, but instead have a logistical ponderment - For how many generations to the Aeslin keep their holidays? when the colonies branch off, and begin creating their own new holidays, do the new holidays begin overwriting or supplanting the old holidays so that there is only one or just a few on any given day? Or do they stack, becoming almost an "on this day in history" sort of situation? If there are multiple celebrations on one day, are the celebrated concurrently? consecutively? Do colonies ever rejoin each other, or cross-pollinate in some fashion (an Aeslin exchange program?) and thus share holidays? or are the new colonies more like religious schisms, and ne'er the twain shall meet?"
I decided that I would answer one question about the Aeslin mice this round, because while I love them, they're sort of like bacon: a little bit can go a very long way, and we're way too early in the series to be risking mouse burn-out. This one offered the most opportunities to stick knives into people, so...you're welcome, I guess.
First off, there's a major underlying assumption buried in this question: the assumption that colonies branch off. They used to, but that doesn't happen anymore, because branching really happens only when the population gets too large for the space and resources available. The colony of Aeslin mice currently living with the Price family is the last known Aeslin colony in the world. The elders control birth rates and expansion very carefully, and pray for the younger generation of Prices and Price-Harringtons to marry and settle in homes of their own, because they're trying to avoid an actual schism; they know very well that any groups that leave the family home are extremely unlikely to survive. At the same time, if a schism becomes unavoidable before a new attic or basement or guest bedroom becomes available to them, the schisming mice will no longer exist from the perspective of the colony. Reject the colony, you reject the colony's gods. Reject the colony's gods, reject the colony's way of life. Reject the colony's way of life, you are no longer my child.
Aeslin mice are pathologically religious. They can't fight the urge to worship. It's tied to their survival instincts; while a colony that worships a cat is likely to be eaten, a colony that worships a tree will have a stronger tendency to stay together and stay safe, because they need to be healthy to properly tend to the needs of their god. They're capable of teamwork and very complicated thought, but they're still mice. Talking mice. The Covenant wiped them out easily as sports of nature and demonic imps. People who found them in their homes captured them and sold them to circuses or traveling shows. Cats, dogs, foxes, snakes...it's a big, scary world for an Aeslin mouse, and it's entirely possible that the colony found by Caroline Davies, mother of Enid Davies (later Enid Healy), was the last one there was. She saved them. She gave them something to believe in.
She gave them her family.
Now, on to the more time-based questions. "For how many generations do the Aeslin keep their holidays?" For as many as they keep their faith. If they worship a tree, then hundreds of generations could pass before their god withers and dies. If they worship a mayfly, they'll need a new god by the end of the summer. The Price family Aeslin still celebrate the Sacred Ritual of I Don't Care What You Say, They're Harmless Little Things and They Need a Home, They're Not Monsters, They're Mice, better known to the family as "the day Great-Great-Great-Grandma Caroline found the mice in the barnyard." Nothing is ever forgotten. Nothing is ever forgotten. To forget anything would be to shame the gods, and to be less than Aeslin.
The Aeslin calendar does not exactly match the human calendar; it has more months, for one thing, and the number seems to increase periodically, although no one human understands how or why that happens. While the feast days and celebrations will always match up to their original places on the human calendar, how often they are observed is determined by a number of factors, including their place on the Aeslin calendar, how resource-intensive the observation is, and how much they like the festival. (The Festival of Giving a Mouse a Cookie, way more popular than The Remembrance of the Violent Priestess, Who Never Learned to Be Careful.) They can, and will, perform any liturgical rite on request, but when they come around naturally doesn't follow a human logic pattern.
The mice who travel with Verity, Alex, and the others aren't considered new colonies; they're still part of the central colony, and will remain so for as long as they share gods. The mice very much enjoy coming back together to consolidate their observances of the family, share rituals, and remind themselves that they are still united.
As long as there are Prices, there will be Aeslin.
The same is not quite as certain in reverse.