What day is Thanksgiving celebrated in Europe?

There are many harvest celebrations in Europe, but they're not called "Thanksgiving". Some of the more well-known harvest festivals are:

• Martinmas (France and elsewhere), November 11th.
• Oktoberfest (Germany), third weekend in September to first Sunday of October.
• Erntedankfest (Germany), first Sunday in October.
• St. Leopold's Feast Day (Austria), November 15th.
• Obzinky (Czech Republic and Slovakia), late August or early September.
• Dożynki (Poland), coincides with the autumnal equinox.

The Oktoberfest is NOT a variant of any thanksgiving celebration or harvest celebration. It was started on occasion of a royal wedding (Bavaria was a kingdom till 1918) to have the subjects partake in the joyous celebrations.