Southwest periodically make flights for further in the future available. You can now book flights through April 11, 2021. Southwest will release the next schedule extension on November 12, 2020.
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’.
Southwests schedule is so subject to change that booking out any earlier than the next 45 days is a waste of time. The direct flights get canceled, connection flights get moved 2-4 hours.
At least they’re as flexible with us as they “ask” us to be with them… no other airline (before the pandemic) had such an awesome cancellable fare.
As much as booking well in advance may lead to schedule changes (as with any airline), at least Southwest seems to have the best rates early on. Less games to play.