Since I can’t control what USPS and its international partners do, I send holiday postcards from about 5 weeks before the holiday until 2 weeks before and hope for the best.
I like to surprise Postcrossers by sending cards for less famous or American holidays if I can find postcards. I’ve sent cards for St. Patrick’s Day, Thanksgiving, and Halloween.
The main holiday cards I send are for Christmas/the winter solstice. I send my international cards about 1 month in advance. Within Canada but outside my province, 3 weeks in advance. Within my province but not local – 2 weeks. And locally, just a little later than that. That way they should arrive a little before the day in question, but not so far in advance that it gets weird.