I'm writing this on Thanksgiving. It's a holiday we enjoy in the States every November, to commemorate the Pilgrim's survival of their first year in the new world. And, as usual, I wonder if any of my various universes would have a similar holiday.
Let's concentrate on Thanksgiving; it is an autumn holiday with lots of food and reflection on all the things we are thankful we have. Before the Pilgrims, a lot of cultures had a harvest feast. The hard work of raising crops was done, and (some) people could now 'coast' through the winter until spring, when it was time to plant and hope for a good crop. But keeping a bountiful crop stored all winter was hard; mice and mold and who knew what else could claim a good portion of it, so feasting might continue well into the winter, until most of the food had become a layer of fat on the people, which would help them survive those last few weeks until they could get their hands on fresh food again.
Mac (MacOnFireball.blogspot.com) definitely does have a Thanksgiving, because the Fleet follows the majority of the Earth traditions, but whether it is called Thanksgiving, Harvest Day or something else, I haven't decided.
In my Atlan universe, a 'god' (alien) brought 3 infants to live on a remote island, and stayed with them until they could take care of themselves. This island did not have seasons, so there would have not been any harvest time to celebrate. Later, after the home island was destroyed, and a few Atlans were scattered about the planet to establish tribes, each tribe might have a day of celebration. Some might celebrate the day they made landfall, or others - who now experience seasons - might celebrate a successful harvest.
In my Tunad universe, I think they might have 2 holidays that resemble Thanksgiving. Their colony is established in early spring, and they celebrate the anniversary of that day. They would also celebrate the first fall harvest, which proved they could raise food on a planet that was much colder than their home world. But the Tunads were very careful choosing holidays as they set up their calendar. They did not hang on to any holidays from their home planet, as they have set out to escape from that culture. Still, they recognize that holidays are important, a way to say, 'Our hard work has paid off,' and 'See how far we've gotten.'
Hope you had a great Turkey Day.