ответьте на вопрос, надо when and why americans began to celebrate thanksgiving day?

Когда и почему американцы начали отмечать день благодорения ?

4 Thursday of November - note
Every year, every autumn in America celebrate Thanksgiving Day (Thanksgiving Day). In the fourth Thursday in November at the holiday table are going to relatives and friends, remembering all the good things and good things that happened in their lives. This tradition originated back in the XVII century, when the New World from England arrived pilgrims, and began a new phase in the history of the Americas.
In 1621, this day was first celebrated in the United States British colonists and the Indians, on lands which was a settlement of the British. Then the meaning of the festival is to raise the spirit of the colonists in the harsh winter on their new lands. Over time, the festival has acquired the elements of religion and became an occasion to thank God for the blessings he sends people.
Summit Thanksgiving Day was recognized in 1789, became a public holiday, approved by decree of President George Washington. To date, noting Thanksgiving Day, Americans are grateful to their fathers pilgrims.