Since the Steam Summer Sale did not start on July 4th last week—which would have been almost too serendipitous, if there is such a thing—plenty of rumors have been flung around claiming that it should arrive this Thursday, as it always begins on Thursday. Considering that it started July 12th last year, it wouldn't be too outside the box.
But rumor has it that several people have been emailed the following message from a Russian company named xSolla, which assists with Valve and Steam on a payment system in the CIS market (view partnership here near the bottom of the page):
Летняя Распродажа на Steam стартует 11-го июля! Подготовься заранее, пополни кошелек Steam *** прямо сейчас - и получи возможность покупать игры со скидкой до 75%!
Translated into English via Google Translate, it says:
Summer Sale on Steam kicks off on July 11th! Prepare in advance, recharge your wallet Steam *** right now - and get the opportunity to buy the game at a discount up to 75%!
So get ready on Thursday to buy an obscene amount of games, half of which you'll play and half of which will become a part of your stack of shame. Don't deny it.