Unlike the legacy airlines, Southwest (and JetBlue) operate their schedules on a periodic extension model instead of a rolling schedule. That means that every couple of months, they will add a couple of months to their schedule.
Southwest Schedule Extended
In the case of Southwest, since you are able to change your bookings at no penalty and even rebook at lower prices, it makes total sense to lock in travel for holiday periods when the schedule opens and then change the booking later if the price drops. But, chances are for holiday travel, you will never get a better price than right when the schedule opens (since fare sales normally exclude the dates around holidays). That means the time to book is now!
The Southwest schedule now runs through January 7, 2018. This is perfect for locking in travel for some of the busiest travel periods of the year – Thanksgiving, Christmas, and New Year holiday periods.
As I checked random city pairs, I found that prices for travel on dates around the holidays to only a little higher than normal prices. Of course, dates like Christmas Eve and Christmas day have the cheapest prices around those periods. If you are only a short flight away from family, this could work!