I actually did some research on this because when I was young the day after Thanksgiving marked the start of the Christmas Season. It seems that in countries that don't have Thanksgiving that some earlier dates are common.
In the US however the tradition has been and still is for most people, to start after Thanksgiving. Commercialism however requires a theme for sales etc. It seems that most stores don't make a lot of profit on Thanksgiving so right after Halloween they move into the Christmas preparations as that is their most profitable season. Some large department stores go into Christmas mode right after the "Back to School" season.
So while most people do not move into the Christmas festivities until after Thanksgiving big business starts priming us in hopes of increased sales in October and some as early as September.
Personally I think it drags the 'season' out and makes it less special to those who only think of it from the "Commercial" viewpoint. And is offensive to those who are involved spiritually.