As is tradition, the starting date for the Steam Summer Sale has been leaked multiple times over the past few weeks. The biggest and most credible of these leaks was also the most recent one as PayPal announced their promotional deal a little bit ahead of time.
With all of that in mind it probably won't come as a surprise to hear that Valve has now officially confirmed that the Steam Summer Sale is coming today. To be more specific, the Steam Summer Sale will last from June 22nd all the way until July 5th.
While I can offer you very little else until the sale itself starts, it is worth repeating that the deals will not rotate throughout the sale like they did in the past. In other words, if you find a game at a deep discount, feel free to grab it as you most likely won't be getting it any cheaper until a few months down the line.