Valve doesn't bother announcing its major Steam sales these days. Maybe the company has plans for big reveals and flashy trailers, but sure as night follows day, someone ALWAYS leaks the start date ahead of time. If it's not Paypal then it's some other third-party site, which is the case this time around with Steam Database revealing a June 21 start date for this year's Summer Sale over on Twitter.
The site in question collects information relating to Steam for players and devs to analyse, and as someone has written on its Twitter page, they "occasionally make the pipes leak." Make of that what you will.
Therefore, take this with the obligatory pinch of salt. However, if the date is correct then we can expect the annual two-week sale to start on June 21 and run through until roughly July 4/5.
Are there any games on your radar that you've been waiting to pick up during the sale?