Speaking to three developer sources today, Kotaku UK has learned the dates and times of the next three Steam Sales. According to images provided by these developers, the next three sale dates are as follows.
Steam Halloween Sale: October 26th 2017 - November 1st 2017
Steam Autumn Sale (over Black Friday): November 22nd 2017 - November 28th 2017
Steam Winter Sale: December 21st 2017 - January 4th 2018
The sales are expected to start and end at approximately 6pm UK time on each of those dates, with a few minutes variation from one to the next.
I don't know about you, but I'm ready to lose a whole bunch of cash on Steam yet again. Bring me all them sweet discounts.