"Today, many of our public school children are taught that we celebrate Thanksgiving because the Pilgrims were thanking their Indian neighbors for helping them; but the evidence of history shows that on that first Thanksgiving Day the thanks of both Pilgrims and Indians went to God for His great goodness toward them all.  But the story does not end there.

Even though the Pilgrims hosted the first Thanksgiving dinner in America, the holiday itself actually has its origins almost 170 years later, after the Revolutionary War had been won and our American Constitution had been adopted.  In 1789, Congress approved the Bill of Rights, the first 10 Amendments to the Constitution.  Congress then “recommended a day of public thanksgiving and prayer” to thank God for blessing America.  President Washington declared November 26, 1789, as the first national day of prayer and thanksgiving to the Lord. 

Another 75 years later, after the Civil War ended, President Abraham Lincoln established the last Thursday in November as a day to acknowledge “the gracious gifts of the Most High God” bestowed upon America.  Every president did the same until 1941 when Congress officially made Thanksgiving a national holiday." 

(from jesus-is-savior.com)