Most airlines set their flight schedules between two and four months prior to the flight date. Most airlines also publish tentative flight schedules 330 days prior to the flight, but Southwest does not -- they only publish their final flight schedule. I'm not sure what protocol for that that Southwest follows -- it would be interesting to know if anyone's been tracking it for long enough. It seems to me, though, that they release flight schedules every two months, for the period 105 to 150 days after the flight schedule release date -- but that's just a guess.