The pilgrims may not have eaten any turkey at the "First Thanksgiving" in 1621—no mention of turkey is specifically mentioned in Edward Winslow's account of the feast, but he does mention that a lot of venison was eaten, and they probably ate fish and shellfish as well—so why is it the traditionally accepted main course for Thanksgiving today? Well, the Week explains, William Bradford's journals were rediscovered and reprinted in 1856, and he wrote about the wild turkey hunts the colonists engaged in in 1621. Plus, turkey is a very American bird, and a large enough one to feed a table full of people, so when Abraham Lincoln officially made Thanksgiving a national holiday in 1863, that's what was on the menu.
The pilgrims didn't eat cranberry sauce, either, considering sugar was a luxury at the time and it's kind of an integral ingredient in the sauce. (They may have eaten plain cranberries, though; National Geographic notes that the fruit was a sort of Native American "superfood.") And neither sweet potatoes nor white potatoes were around in America quite yet, so those weren't on the menu either, nor was pumpkin pie—since the pilgrims probably didn't have the butter or flour they would have needed to make the crust. If you'd like to be historically accurate at least when it comes to the beverages, the pilgrims probably drank hard apple cider, the Daily Beast reports.