A Reddit user by the name of MrFreemanBBQ may have possibly leaked the dates of the Steam Summer Sale this year.
Having leaked other Steam Sales dates before, MrFreemanBBQ posted an image showing that the Steam Summer Sale 2016 will run from the 23rd through the 4th with the months written in gibberish. However, Steam Summer Sales have traditionally gone from June through July, along with being pretty close to the dates and time frame of the 23rd through the 4th. It’s safe to say that, even if the dates are wrong, that they’d be pretty close to the actual dates.
Probably best to wait for confirmation from Valve, though, for the confirmation on those dates.