I'm going to break up with my boyfriend the day before thanksgiving
What's so important about that day?
It's the best day to enjoy myself without him
Do it before that point. I think if you do it according to plan, it feels very premeditated and he'll feel much worse.
I'm going to do it anyway.
I understand if your relationship is coming to an end, but I think it's wise to wait until after the holiday. The both you deserve one more happy memory together!