I don't think there's any way to really plan it ahead, either. I mean, what if the Sox are on a 12-game road trip next April? You never know.
It's a great idea, but I think it's one of those things that has to wait at the very least until the Red Sox schedule comes out. Then, you see if they're playing an afternoon game on a weekend, and you hope that the Celts are playing that night.
Roughly, playoff games start the same time every year, but it's hard to tell when a team will play its first playoff game (ie, could be Saturday or Sunday, etc.)
One thing to remember is that the Sox are always in town around Patriot's day (April 20 next year) so that may be a good bet for you.