The comical thing about anti-colonial backlash aimed at Thanksgiving is that the annual holiday in the United States was actually created to give thanks for the Union winning the Battle of Gettysburg (something I for one am still thankful for) and generally to promote the idea that the future was hopeful as 1863 came to a close. And even that is just the political circumstances that allowed activist Sarah Joshepa Hale to finally succeed at her effort to have the disparate local thanksgiving harvest festivals in New England (celebrations which, despite geography, had nothing to do with Pilgrims) made into a national holiday (with her preferred menu!).
The "First Thanksgiving" with Pilgrims and Wampanoag was an actual event, but it almost certainly took place earlier in the season (during the harvest!) and only became conflated with the annual holiday later. I would presume the conflation happened in part to sell the celebration in the reintegrated Southern states and in part because of general mythmaking and desire to make a newish holiday look like an ancient tradition, but it seems it was mostly the standardization of grade school curricula in the late 19th and early 20th centuries that really put the Pilgrims in the spotlight. Partly this was part of the assimilationist and national pride inculcating agenda of those curricula, but in defense of the school teachers I'll say that a simple story about sharing and survival from early in the country's history makes pedigogical sense for elementary students, whereas trying to explain to students why they should be thankful for Union war victories (long before they study the Civil War) does not.
We were getting in so much Christmas stuff before Halloween that we had to start putting it out in my store's seasonal section. One aisle in seasonal had Halloween candy and paraphernalia, the other seasonal aisle had Christmas stuff. 
