All signs are pointing towards the Steam Summer Sale starting on July 11th... wait. Tomorrow. I just felt a great disturbance in the force, as if millions of wallets suddenly cried out in terror.
As reported by none other than HUKD [thanks Noisetank], a Left 4 Dead 2 fan reportedly received an email from Valve support explicitly stating that "our Steam sales are starting on July 11th." Here's an image of the email, which should perhaps be taken with a pinch of salt.
However, that's just the tip of the iceberg, and the anecdotal evidence matches up nicely. This week's midweek madness sale comes to an end at 10 AM Pacific time tomorrow as opposed to the customary 4 PM PST, suggesting that Valve might want to clear the decks for a major content update. Finally, an advert posted via a Russian company (via Reddit) claims that the summer sale will run from June 11th to the 22nd.
Steam traditionally throws their summer sale in early-to-mid July, so the date feels right. They're also fans of giving no warning whatsoever. Regardless of when the hammer drops, we'll be sure to keep you posted.