Last season the general sale began 8th March; there was a pre-sale for incumbent STH on 23rd-26th February. It'll probably be sometime round those dates.
I've been meaning to make a spreadsheet of all of the offseason announcements and the dates on which they occurred in past years. When are STH packages mailed out, when are single game seats available, when is the spring training schedule announced... I'll try to work on this at some point in the near future.