It's a simple question. In the US, it's the fourth Thursday in November. You'd think that'd be easy to figure out. But apparently it's difficult.
This year, for instance, because November starts on a Friday, the rest of Thanksgiving weekend, from Friday on, is the fifth week of November. This seems to have confused a lot of people into expecting Thanksgiving on the fourth weekend, i.e. the 21st instead of the 28th. (Remember, remember, fifth weekend November / football games, leftovers brought / I see no reason the Thanksgiving season / Should ever be forgot.)
For instance, the glossy, attractively-designed, nicely-printed holiday garbage and recycling collection schedule that the city just sent out on a large postcard. As usual, collection services for Thanksgiving, Christmas, and New Year's, and the rest of the days of that week are all pushed to the following day. That much is on their web sites, but the specific dates for this year are only on the postcard, which says "Thursday, Nov 21 -> Friday Nov 22; Friday Nov 22 -> Saturday Nov 23." Oops. I hope a sufficiency of people are phoning them up to say, "You clowns, didn't you check a calendar?"
But checking a calendar may not help! My pocket calendar for this year has it right, but I was just transferring info to next year's and discovered that it tells me that Thanksgiving will be on Tuesday, Nov. 25.
The prize for awesome stupidity in calendar-making, however, goes to a decorative appointment book I once saw which put 31 days in June and made up for it by omitting July 6. Even Pope Gregory would have been baffled by that one.