do it now. i put off breaking up with my ex for ages: i was always finding excuses for our relationship and reasons to stay together. before i left uni for christmas, i realised that i was just being a coward and that i was staying in a relationship that i didn't want simply because it was easier than breaking up with him... and then i decided to postpone the actual break up until the beginning of this term.
to cut a long story short, being with him when i realised i didn't want to be - even though i wasn't actually seeing him - drove me up the wall and i dumped him two days before christmas. and i don't feel bad about it because, to be honest, i would always have been able to find an excuse about the timing, and it just had to be done. if you don't want to be with him, just end it, regardless of plans: it's better for both of you in the long run.