That's when the bombs dropped, so you generally see a lot of Halloween decorations in prewar areas. But I don't think I ever checked the calendar first thing in the game to see what day/month a new playthrough starts.
I know in Diamond City it will change depending on the real life date I've heard haven't seen it for myself yet but I do plan to be in Diamond City on that date. Haven't gotten that far yet and haven't played the game in a while but I know I got 1 more day to Halloween.