Although individual practice varies by family and town, the typical Thankstravaganza goes something like this:
1. Gathering. Get together a whole mess of people, including ideally both people you like and people you can't stand. The whole point of Thankstravaganza is that "we're all in this together" - it's a community holiday, so invite your nosy neighbors and that guy who borrowed your good plow and never brought it back. Being invited to someone's home for Thankstravaganza is not forgiveness for something you've done, and only a fool would think otherwise. Critical mass for a good Thankstravaganza is twenty people or more. If you can get a priest to attend, that's grand. Some villages or towns manage to get several (competing) clerics in one place, which is always interesting later.
2. Gratitude. As we all know, the Wampus Country is thick with gods, lesser gods, things that act like gods, things that wish they were gods, and things morons think are gods but aren't. Thankstravaganza is a time for expressing gratitude to all of these. Usually the patriarch or host will make a general speech about a mess of stuff for which to be thankful, and then each member of the party is expected to say a few words thanking a particular deity, helpful fairy, or personified weather manifestation (you hear a lot of the latter if the harvest was particularly good that year). Once everyone has said something, the host will either give a final speech or defer to one or more priests in attendance, but the summation is always the same - giving thanks to every being and creature that might've been overlooked or forgotten in the previous expressions of gratitude. Wording this speech to include everything is considered pretty important, lest some lesser, forgotten, or unknown godling ("Itchy Fernando, the Vermilion Burning, Lord of Underpants Rashes") happen to be listening and become offended. Even the lowliest godling is not someone you want to offend. 
3. Gorging. Now, the feast. In some cases the host provides all the food, but it is more usual for everyone to bring something. Fowl, beef, pork, vegetables, grains, breads, cakes, pies, everything. Enough to feed an army. If the guests manage to finish everything presented, you've done something horribly wrong; it is a sign of largesse and community fellow-feeling to over-provide. While the guests are eating, it is customary for them also to quaff ridiculous quantities of beer, wine, ale, mead, and whiskey - sometimes all in the same glass. 
4. Games. By now everyone should be sloppy drunk, belligerent, and ready to vomit at a moment's notice, which means it's time for group physical exertion. While the younger children cavort unsupervised, the adults begin to arrange themselves in teams for the annual games. The community comes together in "friendly" competitions - the individual games will vary, but it's typically something outdoors and rough, like rugby, flaming caber toss, skunder-ball, or Throttle-the-Goat.  After an hour or so of this, presuming optimal levels of participation, everyone at the party will have either vomited, passed out, disappeared, cheated on their spouse, succumbed to a mysterious head injury, or been cheerfully shanked by one of their neighbors over an imagined slight.  Thus is the social contract renewed.
Eventually everyone wanders home and regrets attending, looking forward to the next year's festivities.
 This is a serious warning. More than one village or family line has ended up cursed by something ridiculous for want of including a simple "and thanks to every other being, seen or unseen". The Moorblatts of River-Town all have carrots for noses, remember - you've seen the pictures in the Gazette. I've no idea how Blanchard Moorblatt is going to get his daughter Beryl married off now, which is a great shame, as she is otherwise quite fetching. Perhaps there is truly someone out there for everyone, and Beryl will one day soon find the vegetarian or snowman of her dreams.
 In Saltvale, it is traditional to serve at least one round of "Shazinger's Widowmaker", a concoction blending mushroom-beer, oak-aged whiskey, and lamp oil, served warm. Ernst Shazinger himself was a serial adulterer and philanderer who murdered his paramours' husbands by soaking them in oil and setting them alight. Once his crimes - and nocturnal meanderings - came to light, his own wife punished him appropriately, which is why a glass of honey-mead with sheep testicles in it is sometimes referred to as a "Mrs. Shazinger".
 If you need ideas for games to play, check out this random table of amusements. Skunder-ball involves throwing surprisingly-hard chestnuts at another man's crotch at high speed and short distance; "Throttle-the-Goat" is a simple variation on the classic and well-known goat-choking games we all played at school, the main difference being the addition of darts, blindfolds, and the oldest man in town with his pants down playing multiple harmonicas.
 You'll note six options there. It's a hidden random table.
|There is zero chance I'm putting this here so I can find it again for Friday night's game.|
It's a "turkimera", by the way. Don't speculate further, you'll ruin it.