Since Southwest completed its purchase of AirTran the two tend to team up on sales. To get these fares, you must purchase by Oct. 20 for travel Nov. 30-Dec. 14 and Jan. 4-Feb. 15. Southwest requires a 14-day advance purchase and blacks out Sundays. Here is the fine print for Southwest and AirTran. You'll notice that these discounts do not apply to the holiday periods. Southwest ran a holiday sale earlier this month.
So two last things to consider: Late fall tends to be a slack time for leisure travel so if these two are putting up sales there may be competition on selected routes. And second, if you plan to travel in the holiday season you'll need to keep an eye out for sales and jump on them. I've seen forecasts that suggest the holiday season may be a little softer than last year so some price movement wouldn't be a big surprise.
UPDATE 1:15 p.m.
JetBlue has just leapt in with a two-day sale for travel between Oct. 25-Dec. 15. Sale fares are valid until Oct. 18 at midnight, with blackout dates of Nov. 18-28 and all Fridays and Sundays. So this is competitive with Southwest-AirTran and neither helps you out if you're looking to travel for Thanksgiving. Here is the fine print. Still competition is a good thing.