When did people celebrate thanksgiving day for the time and why

Thanksgiving day is an old holiday in the USA. It originated there and is marked only in this country. Americans celebrate it annually on the 4th Thursday in the last month of autumn. Its history dates back to 1621. Having got the rich harvest, the Pilgrims marked the success with a feast of gratitude. Since those old days Thanksgiving has been marked as an American national custom when every family with relatives and guests gather together to have feast dinner. Traditional roast turkey with fried potato, cranberry jelly and, of course, pumpkin pie must be on the dinner table.