Thanksgiving Day. It's a holiday Americans think they know how to celebrate. As every school child learns, there was a feast in the autumn of 1621 at the settlement of Plymouth, Mass., a festivity that brought together Pilgrims and Massasoit Indians heralding the end of a severe winter.

But, according to Massachusetts historian Diana Karter Appelbaum, the specifics from there on resemble only faintly what has become the traditional version of Thanksgiving Day. America's best-known feast, it seems, is only part fact -- with hefty servings of legend and fiction.

For example:

* None of the Pilgrim's historical records suggest that turkey was served -- although along with partridges, ducks and geese, they were among the fowl native to the Cape Cod Bay region.

* Accounts show that venison and probably fish, oysters and clams were entrees.

* There was no bread nor pumpkin pie. Flour supplies brought over from England had long been depleted, and wheat harvests in the tough New England countryside were years away. Pumpkin was probably served like squash, as a vegetable.

* No apple cider, milk or cheese made it to the table. Cows weren't part of the Mayflower's hold. There was plenty of corn, which the Indians taught the colonists to grow. And cornbread. The availability of fruit and nuts suggests the feast may have included cranberries.

Our popular misconceptions about the original Thanksgiving are not limited to the dinner table, says Appelbaum, 31, whose new book, Thanksgiving: An American Holiday, An American History (Facts On File, $15.95), attempts to defy tradition and myth to discover the Thanksgiving behind the holiday.

The facts she uncovered, she says, typically intrigue and disturb most people. "My reaction was disappointment . . . My mother refused to believe it."

The book's most unsettling contention is that the forerunner of all Thanksgiving days was never intended to be an annual holiday and, by definition of the times, shouldn't even be called Thanksgiving. A synthesis of European traditions, the pilgrims' historical fe te was more like an Old World "Harvest Home" celebration -- a day of drinking, recreation, even sexual shenanigans -- than a "thanksgiving." It's a distinction Puritans and purists certainly made.

"The Puritans' concept of a thanksgiving day was that it was a religious day . . . to give thanks for some specific event, like the end of a plague," says Appelbaum. "It was a day of solemn prayer and devotion. There might've been two or three in a good year." In fact, the Pilgrims' feast included politicking, hunting and turkey shoots with the Indians.

Some other trivia plucked from Appelbaum's book to spice up your day:

* Thanksgiving eventually became recognized as an annual celebration by the 1640s because of a scarcity of holidays. Strict Puritanism had banished Christmas, Easter and All Saints' Day from the calendar as both too Roman Catholic and laden with pagan practices. Well into the 18th century, New England's only holiday excitement was Muster Day, Election Day and the Harvard Commencement.

* Credit the birth of the holiday itself to colonial Connecticut, first to proclaim it an annual festival (Sept. 18, 1639), in gratitude for general blessings. Records for the decade that followed are incomplete, but since 1649, the people of Connecticut have failed to observe Thanksgiving only once: in the midst of bloody warfare with the Wampanoag King Philip's Indian warriors in 1675.

* Many prominent citizens of the time argued publicly against a yearly celebration tied to no particular blessing. Judge Samuel Sewall (later a judge in the Salem witchcraft trials) complained it would "make the people overly confident of the Lord's generosity."

* During the late 1600s and early 1700s, Massachusetts, New Hampshire and Connecticut enacted laws to fine participants 10 shillings for each offense of "game, sport, play or recreation on . . . Thanksgiving."

Those and other measures failed to dampen youthful enthusiasm. In the mid-1880s, Thanksgiving became the day of the greatest gridiron contests, most notably the Yale-Harvard game at the Polo Grounds in New York City. Traditionalists lamented the intrusion of sport on the holiday; The New York Times suggested in an editorial the name "Thanksgiving Day" be replaced with "Football Day."

* By the 18th century, pumpkin pie had become an essential ingredient. Residents of Colchester, Conn., met in the autumn of 1705 and voted to postpone Thanksgiving for one week because its shipment of molasses had been delayed by a snowstorm. No molasses, no pumpkin pie.

* Why is Thanksgiving on Thursdays? "Puritans observed the Sabbath as a Biblical ordinance," says Appelbaum, and refused to intrude on it with a holiday. Saturdays were for preparing for the Sabbath and Mondays, apparently, were for recuperating. Fridays were overruled because they were the fasting days of the Roman Catholics and automatically carried unacceptable papal overtones.

"Thursdays were the traditional lecture afternoons of Boston ministers, and favorite days for fasts and weekday religious meetings, so they lent themselves to the holiday."

* The first Thanksgiving proclamation by a president of the United States was issued by George Washington in 1789, specifying gratitude for the enactment that year of the Constitution -- but only after lengthy congressional debate. " The people may not be inclined to return thanks for a Constitution until they have experienced it ," objected Thomas Tucker of South Carolina.

* By the mid-1800s, Thanksgiving remained predominantly a New England holiday that attracted hordes of friends and relatives from other states. In 1858, for instance, it was estimated that upwards of 10,000 people left New York City to travel north for the holiday. A few years before, Philadelphia grocers urged Pennsylvania's governor to proclaim the state's own observance: The mass November exodus to New England was bad for business.

* More than any other individual, a New Hampshire widow named Sarah Josepha Hale is credited with making Thanksgiving a national holiday. As editor of The Lady's Book, a predecessor of magazines such as Redbook and Good Housekeeping, she started her state-by-state campaign in 1846 to make the last Thursday in November the national Thanksgiving Day. In 1863, she finally convinced President Abraham Lincoln to proclaim the fixed national holiday.

* When President Roosevelt bent to pressures from national retailers in 1939 to move up the date of Thanksgiving to create an extra week of shopping, it threw the holiday into chaos. Individuals, cities and states observed the holiday on different days in November, depending on which side of the national controversy they stood. Traditionalists and Republicans were outraged, and so were football coaches whose game schedules were suddenly awry.

In 1941, FDR restored the holiday to the fourth Thursday in November.

You're at least convinced that today's holiday began in Massachusetts? Four other states -- Texas, Maine, Florida and Virginia -- claim the honor of hosting the first Thanksgiving.