On Thursday, Americans will gather to celebrate Thanksgiving, and they will do so at the dinner table. The meal, with roots in the ancient traditions of harvest festivals and in the religious fervor of the Puritans who settled here, has grown into a secular feast of abundance. Whether taking place in Duluth, Minnesota or at the Bagram Airfield in Afghanistan, the American tradition is to roast a turkey and eat it along with a slew of side dishes, all in the company of family and friends.