We flew Southwest for for the first time this past Christmas for our WDW trip...it was great! The fares for our trip (12/18 - 12/25) came out about the second week of August...I'm pretty sure the dates for early December were out before that, I just can't remember how much earlier...sorry! We felt SWA offered great service...the flight attendants were really friendly...even on Christmas day when we flew home. Just remember if you fly with them to check in online 24 hours prior to your flight...and when at WDW, at the 24 hours prior to time...just ask the front desk to check in for you...you'll have to be present for them to do it, but it was easy enough and they were happy to help.
Have a great trip!