At some point Valve have presumably given up and put Paypal in charge of announcing when the Steam summer sale will begin, since the company consistently manages to leak the date ahead of time.
According to Paypal Steam’s summer sale will begin on June 23rd. Thus far all previous leaks have proven accurate.
This seemingly confirms a post by a developer on Reddit who put up an image showing the start and end date for the Steam summer sale. Clearly he didn’t read the bit that said “confidential.”