Converting Thanksgiving into a Citadel submission shouldn't be that difficult. And to prove it, here are the first 5 ideas that came to mind, and who knows, they might be enough to get a few imaginations in gear.
1. Small Town Festivals - rather than emulate the US national holiday, share the spirit of it, the coming together of people to celebrate their bounty, or their simple survival. It is only in later years that the holiday became the gateway to Christmas shopping. In days past it was a more insular time of community celebration and family gatherings. For example, the community of Forsynwaith was almost destroyed by a famine 200 years ago, but the timely assistance of the elves and their waybread saved the community. In thanksgiving, on the anniverasary of the famine, they share loaves of bread, some artistic, others pastry, etc to remember the gift of the elves.
2. Turkey from Hell - in typical twisted fashion, this lifeform is a monsterous turkey that lives only to devour people and than laze about for the weekend. Can the PCs defeat this giant beast, save the village, and then get to carve into its succulent white meat?
3. The Order of St. Benjamin - Benjamin Franklin originally wanted the turkey, rather than the eagle, to be the bird of the US. In this vein, the Order of St. Franklin considers the gobbler to be a sacred animal, and during the feast of St. Benjamin the birds can be offered to the god/s as sacrifice, but only by an ordained priest of the order.
4. A Plain Feast - the PCs are requested to liven up a boring feast by snaring a unique monster for the lord to serve as his main dish, better yet, two of the things, one to practive on and the other to serve. The animals are better brought in live and as unharmed as possible. The lord would really like to serve a manticore, but you know, whatever they can come up with.
5. In the Thick of it - The PCs play the role of the desperate settler/explorer type and encounter an unknown/reclusive culture that rather than hunt/attack offers the strangers hospitality and food to fill empty bellies. Of course, do the PCs share the favor or do they return home as quick as possible to make way for expeditions of settlers/miners/missionaries to 'civilize' those savages who helped them?