The tweet in question states that the Steam Summer Sale will begin at 6:00 PM BST (that's 10:00 AM Pacific Time or 1:00 PM Eastern Time for you Americans - you can work it out for your location at a site such as thetimezoneconverter.com) on June 22, 2017. Valve has yet to officially announce the start and end dates of the sale themselves.
Aside from the tweet, PayPal is running a promotion where you'll get £5 off from PayPal when you spend £20 on Steam. The promotion will be lasting from June 22, 2017, until July 5, 2017. The dates would coincide with an image that was leaked to Reddit some time ago. Also mentioned in the thread (and important to note) is that the Steam Gifting system has changed somewhat in recent months - it'll be a good bit more difficult to take advantage of regional price differences and send gifts to friends in other countries if the values are wildly different.
Steam sales used to be flashier affairs with daily deals, shorter flash sales, and other interesting ways to shave off a few percent off of games that the patient gamer has waited for, but the Washingon-based game developer and digital distribution company has simplified things in recent years. It's something I wasn't fond of at the time (and a colleague somewhat disagreed). In any case, if the tweet holds true then you have just over 24 hours to brace your wallets for the onslaught.
Are there any particular titles you're looking forward to picking up during the Steam Summer Sale? Do you buy something during any sale, or do you wait until the Winter Sale to get the best deals? Do you think the 2017 Steam Summer Sale will actually kick off tomorrow? Let us know in the comments below!