The dates for the sale came from Valve's official social networking platform in Russia; VKontakte. The sale is said to end on 20th June, after which another two days of encore sales will be held. The encore sales will include the top offers from the entire sale.
For those of you who are wondering what a 'Steam Summer Sale' is; it's a wondrous time when the Steam PC gaming platform offers up a ton of its games at, usually, ridiculously low prices. The sales often have games on offer for limited time periods, which then switch over to other games when the time is up. Previously there have also been meta-games, which give purchasers points or trading cards that can be combined into other rewards. Steam basically offer up an amazing sale that, if you're a PC gamer, you really wouldn't want to miss.
All that being said, you may end up like me, owning hundreds of games on Steam that I either haven't played or are yet to finish, mainly because of Steam Sale impulse buying. But hey, we all need a hobby, right?