It's usually a Saturday in late May -- a few days earlier for the STH waiting list pre-sale.
Those will only be 300-level seats though, and they'll probably sell out within three or four seconds for each game, so you'll have to be very quick. Also, those require you to also buy preseason tickets, and I think there's a 2-per game, 8-per season, per person limit.
You're better off trying TicketMaster or TicketExchange the week of the game. With some luck (also known as checking as frequently as possible), you can usually pick up 100 and 200 level tickets at face value. For a game late last season, I remember somebody getting Row 1 seats behind the visitors' bench. For the Chargers AFC Championship, I got 2 seats in 109, row 9 at face value - around 7:30 AM the Friday before the game on TicketExchange.