Southwest's schedules are updated roughly 7 months in advance. So it is likely that with the November update you'll be able to book flights through the first week of July, while the December update would open reservations through roughly the first week of August.
We are currently accepting air reservations through April 7, 2019. On September 27, 2018 we will open our schedule for sale through June 8, 2019. This date is subject to change! Please check back frequently.
The following schedule release should cover July, 2019, but there's no way for sure of knowing when that may be. Best guess would be sometime in November.