Southwest periodically make flights for further in the future available. You can now book flights through August 16, 2021. Southwest does not have a date for when they will extend the schedule again (this is unusual as usually one is listed).
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’.