There's never a bad time to get out of town. The winter is, of course, a good time to be moving southward, but no one ever says, 'Oh man, I really wish I wouldn't have gone on vacation. I'd much rather be in a dark cubicle filing TPS reports.' That's especially true when tickets are as cheap as they are right now.
American Airlines has $105 roundtrip tickets to San Juan, Puerto Rico flying out of Miami this month. That fantastic price has some limited availability, but there are a handful of dates at that price in late January and early February.
The prices aren't quite as cheap, but there are good deals in other cities as well. You can fly out of New York for $225 and out of Chicago for just $299.