Steam Summer Sale Date May Have Been Leaked
A post on Valve's Official VKontatke, essentially a Russian Facebook, was made saying that the Steam Summer Sale will begin on June 11th and have an "encore" on June 20th.
According to Google Chrome's auto-translate feature, which may or may not be accurate, the post says the following;
"Steam Sale starts June 11 at 20:00 Moscow time. June 20 to begin "Sale Encore," which will involve all the companies that participated in the summer sales. The very sale ends June 22 at 20:00 Moscow time. About events and other details so far unknown. Also we advise to read our guidance on sales, not to spend all the money in one day. # @ steam sale"
This is in line with Valve's past Summer Sales. 2014's sale started on June 19th and 2013's sale started on the 11th. Granted, any number of things could very well destroy the legitimacy of this post, considering there could be miscommunication on Valve's part, but it's a pretty safe bet to have you wallets ready on June 11th.