The schedule could always be updated. Spring is good time to start though. I would say between February and March. Either way, they will steal a lot of SY leisure traffic.
It’s extremely doubtful they’ll change their schedule prior to March 6, especially when the news articles I’ve read said they’ll be starting in March. Please read these Southwest blog entries on how Southwest’s handles its scheduling.
So remember the “120/180 rule” for future bookings on Southwest. We will usually make up to 180 days of inventory available for sale, then allow that booking window to tick down to 120 days-then “recharge” the booking window by putting another 60 days or so of salable inventory out there, taking the total days available back up to around 180. But if you keep this general rule in mind, it’ll provide a good yardstick to see where Southwest stands in with respect to the booking window.