You're probably really stressed out about making dinner, huh? Have a ton of questions about cooking for large groups, vegetarian substitutes, and that deep frying experiment that went completely wrong? What a perfect time for us to remind you that we have a site for that... it's called Seasoned Advice.
Reminding people to turn to Stack Exchange sites at opportune times like this is a really easy way to help promote them. That doesn't necessarily mean waiting around until a holiday - it can be the release of a new operating system, a new video game coming out, really anything that prompts a lot of questions is a good excuse to say "hey, you know there's this thing called Stack Exchange, right?" Excited about the new Twilight movie? Ask about it on Sci Fi. New edition of your favorite journal coming out? Ask about it on one of our academic sites. Need tips on how to work off all that extra stuffing and pumpkin pie you're going to eat tomorrow? Remind your friends that they can ask on Fitness & Nutrition.
There are times and events that correspond with a lot of our sites, and they are all great excuses to do a little promoting. So when you (or whoever is slaving away in the kitchen tomorrow) has a question about cooking Thanksgiving dinner, remind them that they can ask about it on Stack Exchange!