**Update 6/11: The Steam Summer Sale has begun**
If your wallet is already cowering in fear of the upcoming Steam Summer Sale, then you better not give it any indication that you might know the exact dates. According to the Russian IGN site, a leaked date suggests that the sale will start on June 11 and run through June 22.
The leak apparently came from an insider, and the exchange regarding the dates can be seen here. However, it is a rumor after all, so don't take this as a definite. Given that the Steam Sale typically runs in June, it's not be hard to believe.
The translation to the Russian IGN page reads:
Steam Sale starts June 11 at 20:00 Moscow time.
June 20 will begin "Sale Encore," which will involve all the companies that participated in the Summer Sale.
The very sale ends June 22 at 20:00 Moscow time.