According to #Valve’s social networking site in Russia, the Steam Summer Sale is expected to to #launch #June 11th.

The message translates into the following:
Steam Sale starts June 11 at 20:00 Moscow time.
June 20 will begin “Sale Encore,” which will involve all the companies that participated in the Summer Sale.
The sale ends June 22 at 20:00 Moscow time.
The 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.
This means the sale on through the week of E3 2015. Meaning between all the upcoming news and game release‘s, refreshing that Steam page and keeping on top of the current flash sales and daily deals should keep you busy. No pressure.
As of now, Valve has not announced any official dates for the sale, but June 11th is only a couple of weeks away.
