If there's major weather in the forecast, I'd fly out before it hits - even if that means Saturday.

Otherwise Monday, but not the last flight that gets there in time. If there's only one of those, then Sunday, but that's hard to imagine from BOS to anyplace within two hours flying time (i.e., closer than Chicago) that's also big enough to be served by non-stops. If the backup flight is often full, make sure there are two more after the one you book.