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 November 4, 2016.
However, I just saw on the Southwest flight schedule site that on June 2, 2016, Southwest will open up their schedule for reservations through January 4, 2017. This means in almost a month 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.