Southwest, unlike many of the other airlines, does not open up their flights 330 days in advance. Instead, every so often they extend their reservations system a few months out at a time. As of today, you can only book flights through October 31, 2014.
However, I just saw on the Southwest flight schedule site that on May 14, 2015, Southwest will open up their schedule for reservations through January 4, 2016. This means in just two weeks you are able to make reservations for Thanksgiving, Christmas, and New Years.
Those peak holiday flights increase in price quickly, so if you are looking to book a flight then I suggest you take a look at the calendar now and figure out your ideal time in advance. Once the booking system has been extended I will be sure to write about it again.