A lot of preparing needs to be done before a Steam Sale. I like to give my dog my wallet to look after while I flee to a remote, secret location with no internet access. It can be tough predicting when they'll start, however. Thankfully, the next three Steam Sale dates have all been leaked.
Three developer sources confirmed the dates with Kotaku, providing images, so the leak certainly seems to be legitimate. It covers the Halloween Sale, Autumn Sale and Winter Sale.
Steam Halloween Sale: October 26, 2017 - November 1, 2017
Steam Autumn Sale: November 22, 2017 - November 28, 2017
Steam Winter Sale: December 21, 2017 - January, 2018
This should give you plenty of time to start whatever your personal Steam Sale ritual is. They should all kick off around 6pm GMT.
I’d better give the dog my wallet now...