Why does Canadian and American thanksgiving happen in different days?

The Canadian Thanksgiving is truly a harvest holiday, along the line of the Jewish Sukkot (harvest celebration). Which (not coincidentally) starts tonight (10/13).

American Thanksgiving memorializes the anniversary of the arrival of Pilgrims to the New World (November 1620). It has nothing specific to do with the harvest.

Note, too, that the harvest season in Canada ends about a month or more earlier than in the agriculture regions of the US, because of the latitude.

Summary. Different holidays, entirely. But same name.

^The original motivation for the U.S.Thanksgiving may not have been as a harvest celebration, but the way it is celebrated, with emphasis on food that could have been harvested at the time, does make it a kind of harvest celebration. How many people know or care about the Pilgrim connection?

@LostInParadise People with historical interests.

Everyone who went to school in America knows about the pilgrim connection. They make pilgrim hats in 4th grade.
I got dropped in a classroom doing that last year. It took me forever to figure it out, much less teach it.