This year’s Steam Summer Sale will commence on June 23, 2016 at 9:45am PT and will last for two weeks, ending on July 4th.
A screenshot of the announcement made its way to Reddit overnight, which appears to be a message directly from Valve to developers in order to seek approvals for its Summer Sale discounts.
Even though it clearly says “this information is confidential” and requests developers to not post event details anywhere, that doesn’t stop the information to leak ahead of time.
Last year’s Steam Summer Sale took place a bit earlier, which started on June 11 and ended on June 20th with its Encore Sale lasting until June 22. Based on the message released to developers, there appears to be no mention of an Encore Sale, although that could certainly change once Valve officially announces this year’s Steam Summer Sale.