My family uses Southwest so if you do... I'd check online as soon as you could and keep tabs on the lowest air fare and grab the lowest as soon as your vacation timeframe comes available... If you use another carrier... I don't know.
The best deals on SWA are when the fares first open up. SWA releases their next round of fares on Thursdays and usually the second or third Thursday of the month for the next 30 day period going out 6 months.
Flying SWA from PIT to MCO Dec. 17-23. Purchased them pretty much the first day the dates were available. $79 each way. Looked a couple days later and the lowest prices were pretty much gone. So I guess I would book early and hope for a ding later (with my dates I doubt).
Christine