It's Thanksgiving Day in Canada today, but it won't be Thanksgiving Day in the United States until November 27. The fact that these namesake holidays occur in the neighboring countries over a month apart is well known, but the reason why isn't. As it turns out, these dates were chosen by the respective governments. In 1957 the Canadian government proclaimed "A day of General Thanksgiving to Almighty God for the bountiful harvest which Canada has been blessed -- to be observed on the 2nd Monday in October." The day became a federal holiday in the United States in 1863, and is affixed to the fourth Thursday of November due to an act of Congress that was passed in 1941. (Thanksgiving had been observed in both countries prior to this, but the specific date of these celebrations was ever-changing.)
Incidentally, the Columbus Day holiday, which commemorates explorer Christopher Columbus' landing in the Americas, falls on the second Monday in November in the United States.
To learn more about the differences between Thanksgiving Day in Canada and the United States, check out this article over at Mental Floss.