ie: This was not the first official Day of Thanksgiving, as the Plymouth and Massachusetts Colonies had observed such days in prior years. Plymouth had proclaimed Thanksgiving Days for Dec. 20, 1620, in Oct. 1621, on July 30, 1623, in June, 1633, and possibly others as well. They also proclaimed numerous Days of Fasting. Gov. Winthrop of Massachusetts had proclaimed Thanksgiving Days on July 8, 1630, Feb. 22, 1631, June 13, 1632, and June 19, 1633. Various towns in Massachusetts proclaimed Thanksgiving Days as well. In addition, there were numerous Fast Days proclaimed during these first years.
it was just a cherry picked coincedence. Did they 'give thanks' for slaughtering the indians? sure - but they always gave thanks for everything major that happened in accordance with tradition.