It's really hard to tell with SW. Usually it's about 3-4 months out, but for example for dates up to the 1st week of May this year (2007) the fares came out in November! You just have to keep checking. Are you signed up for their email newsletter? That's how I found out they released the fares so early for our May trip, if I didn't get their emails, I wouldn't have even started checking for our dates til Janurary, when I got the email in November for our dates in May, I couldn't believe it!!!!!!
Best bet...especially for Christmas time travel, is to set a price you are comfortable with and when you see it, whether it's Air Tran, SW, or whoever,
just book it!