"[I]n this world nothing can be said to be certain, except death and taxes." The words of Benjamin Franklin that have been echoed time and time again may need to get a little amendment to include Steam Summer Sales. Yes, once again there will be a Steam Summer Sale, and if the leak that came out today is any indication, it sounds like it's going to go down at roughly the same time frame as last year's.
Over on Reddit (first seen by PCGamesN), someone posted a screenshot of the dates which they say were revealed in the closed Steamworks developers group. The dates in question are June 22nd at 9:50 AM PST to July 5th, 2017 at 10:05 AM PST.
Last year, Valve's hotly anticipated Steam Summer Sale started on June 23rd and ran until July 4th, so it makes a lot of sense that the dates would be very close to what they were a year ago.
If you've been patiently awaiting a good deal on those games at the top of your wishlist, or those on the fringe, Steam's Summer Sale is one of the best times to take advantage of great deals. The hard part is bringing yourself actually to play what you buy.