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Related to other Harvest festivities, Thanksgiving dates back to 1621 when the Mayflower pilgrims thanked God for helping them survive the harsh winter when many of them died of cold and famine. I suppose they also thanked the indians that taught them to grow corn and other vegetables.
It was Abraham Lincoln who, foreseeing the importance of a national day that could unite the country, decreed that it should be celebrated on the 4th Thursday of November.
You will get more information in this video.
Glitterfy.com - Thanksgiving Glitter Graphics