A post on Russian social network VK indicates that the wallet-ravaging Steam Summer Sale will begin on 11 June.
The post is apparently from Steam’s official account on the site, and – after running it through Google Translate – it states that the sale starts on 11 June “at 20:00 Moscow time.” The sale proper will run until 20 June, at which point the Encore sale will begin, offering repeats of some of the best deals until 22 June.
All of this seems pretty convincing, but I’m still slapping a bit “rumour” on this because I don’t speak Russian, I have no familiarity with the VK social network, and so I have basically no idea if this is authentic. It looks about right, though, and follows the same conventions as past Steam sales.
You can see the post on VK for yourself over here, if that’ll help you make your mind up about the authenticity. Probably best to prepare your wallets and Steam libraries, anyway, because both of them are likely to be caught in an avalanche of deals.