According to a Reddit user, he has been told by a Steam-Support representative that Valves yearly Summer Sale will start July 11th.
Take this with a grain of salt, but better get your wallet ready to thrown at your display.
If the sales happen likely start at 10:00 AM Valve time
Update 7/11/2013: Steam has this unspoken policy to not release new games during sales. Seeing that there are no releases between today till July 23, that give us a long window to spent a lot of money.
Remember to hold off buying your favorite game until the last day of the sale, unless there is a daily sale or flash sale. You want avoid to see the game you just bought with –50% off next day discounted further by –75% or even 90%.