Southwest periodically make flights for further in the future available. Starting tomorrow (May 30th between 7am and 9am eastern) you’ll be able to book flights until January 5th, 2020. You can now book flights until November 2, 2019. This covers the busy thanksgiving, Christmas and new years periods.
Southwest has a pretty good cancellation/refunds policy, depending on the fare you’ve booked you’ll either receive a credit towards a future flight(Wanna Get Away) or cash refund (Business Select or Anytime – returned to credit card if you used one). That means it’s often best to book flights as soon as they become available and periodically check to see if the price has decreased, if it has you can click ‘change flight’.
Hat tip to SouthFayetteFan