This year's Steam Summer Sale will kick off in late June, if this leak is to be believed.
About this time every year, PC gamers start looking forward to the cornucopia of deals that is the Steam Summer Sale. A screenshot posted to Reddit reveals the start date as June 23, with the sale ending on July 4. The screen shot appears to show a request for discount amounts sent to developers by Valve, and the months appear in Russian.
Normally, a screenshot like this wouldn't be all that reliable, but this one was posted by a user named MrFreemanBBQ, who has a history of getting Steam Sale information very right. He's gotten the dates dead on for the 2015 Summer Sale, the 2013 Holiday Sale, and the 2012 Summer Sale, so it stands to reason that he very well could be right again.
We've reached out to Valve to see if they'll confirm the dates, but I'm not hopeful. In the meantime, join us in the forums, and tell us what games you really want to pick up on sale this summer!