Like many Americans, I am flying off to spend Thanksgiving with family in the US. It is a cultural imperative. No holiday, no festival, is more compelling to my compatriots. By way of a measure, there are more people on the road or in the air than at any other time in the year. They have to go. If they can't go home, they have to find other Americans, or ersatz Americans, to spend the holiday with - which is to say share the meal with. If you have ever been a lone foreigner living in the US at this time of year, you will have experienced another side of that compulsion. People want to bring you home for Thanksgiving. They believe you will be unhappy and lonely if you miss this meal, that you will be, in a sense, without a family and without a nation.

You might well wonder how and why this blow-out meal came to be such a key symbol and ritual for Americans.

The story every American hears is about the pilgrims' first bleak winter in the New World. They found themselves looking starvation in the face when a party of friendly natives arrived bearing local foods - turkey, corn, cranberries. Everybody sat down to a hearty communal meal, properly thankful for God's bounty. It is an interesting tale. A foundation myth in the classic form that explains and morally justifies everything that follows. It gives the pilgrims "indigenous" status through the gift and consumption of local food, and one supposes the Indians got to hear about God through the prayers. Never mind that the friendly Indians were eliminated over the next century.

It is hard to say when the story of the first Thanksgiving arose. For a long time, it only had currency in New England, outside of which the tale did not play well. Nor was it popular among some extreme Protestants for whom any holiday feast smacked of paganism or Catholicism. Those who did celebrate the feast did so on various days.

All that began to change in the 19th century, when a number of states began to adopt the holiday, though still on different days. The crucial push came when Sarah Hale, editor of America's most popular monthly magazine, Godey's Lady's Book, campaigned to turn a holiday she knew from her own youth into a national feast. In fact, Thanksgiving was one of a number of ritual occasions she was concerned with, using them to propagate the cult of domesticity that so marked Victorian America and Western Europe.

Through illustrations, essays, stories, poems and editorials, Hale explored and promoted the domestication of all celebrations and life-crisis rituals. Birth, death and Christmas were all relocated in her magazine from church and community to hearth and home.

There, the ritual dominance of the minister was replaced by that of the wife/mother. Hale was what you might call a ritual feminist, feeling that women's power rested on a moral authority that should be augmented by the control of all such ritual occasions. But in the case of Thanksgiving, the origin myth gave the feast a special symbolic possibility. It was domestic, a family meal, but through the symbols of native foods and the story of pilgrims and Indians, it stood for the nation as a whole.

For the ritual to work properly, Hale reasoned, every American family had to do it, and on the same day. The answer was legislation, so she organised a letter-writing, lobbying campaign to get the US Congress to make the day an official national holiday, to be observed by everyone on the third Thursday of November. In the middle of the Civil War - one can imagine the implications of the timing - she was successful.

We have been eating turkey, corn and cranberries ever since, and indulging in a kind of national amnesia insofar as Sarah Hale is concerned. For part of the power of the occasion is clearly in our collective ignorance of its status as an "invented tradition". We need to believe that it does indeed go back to our national beginnings.

That is why I spent my childhood, like every American, snipping out the black silhouettes not only of Washington and Lincoln, but of pilgrims, Indians and turkeys. And, of course, listening to the story of the first Thanksgiving, before going home to the meal itself. Around the table then, we could feel at once American and yet an exclusively defined family. Both identities could be accommodated by the meal. Immigrants from other lands learned to eat the symbolically American foods, but developed the custom of adding a national food of their own to the meal. All my Italian friends had lasagne with their turkey; the Poles, kielbasi sausage.

But Thanksgiving's role as the definitive family meal made it particularly useful socially. If you wanted to confer an intimate and possibly serious status on a girlfriend or boyfriend, you brought them home for Thanksgiving. Any friend geographically isolated from his or her own family had to be brought home, and foreigners, as I said, offered a kind of special opportunity to demonstrate both family and national values. Yet there is a dark side to Thanksgiving, and an unavoidable one. Its function as the definitive family meal rests on the notion that everyone has one, and only one, family. As soon as you marry, you enter into the holiday combat zone. With whose family will we spend Thanksgiving?

For the children of divorced or separated parents - nearly the norm in the States nowadays - invidious choices begin earlier and with far more painful results. Even within families assembled for the feast, members may be unable to conceal their unfamiliar feelings.

But the food tends to win out in the end. Women (still) produce vast quantities of food, everyone eats far too much, then the men go off to watch football on the television and the women (still) go into the kitchen to clean up. I wouldn't miss it.

Lawrence Taylor is professor of anthropology at NUI Maynooth