Depends on your definition of "mistake." ;)

B-4g+

I didn't know too much of the history of Canadian Thanksgiving before looking it up, but it seems that we in Canada only started celebrating Thanksgiving annually in 1879. Before that, there were years when it was celebrated in May-August, often in response to specific events, like the ends of wars. It was either in October or November for a while, finally ending up on the second Monday of October for most years after 1931, which became official in 1957.

I think the choice of date is mostly due to the fact that the harvest is long over and it is winter by the time the American Thanksgiving rolls around. Our first snowfall is generally in October, with the snow coming to stay in early to mid November (by which point it is regularly ten degrees below, which I think is about 15 degrees F).