Steam sales tend to leak in the weeks and months ahead of their commencement, but this time Valve has simply published them. Sure, the dates are on Steamworks and are intended to be a guide to publishers and developers, but you can access it too.
The documentation confirms dates for the Steam Autumn Sale and Steam Winter Sale, and both are pretty obvious. In fact, we even correctly predicted the Steam Autumn Sale date. But here's when they're confirmed to happen:
Steam Autumn Sale - November 24th - 30th, 2021
Steam Winter Sale - December 22nd, 2021 - January 5th, 2022
Of course, those are just dates for Valve's big sales hoedowns: Individual publisher sales are likely to occur between those, and the previously confirmed Steam Halloween Sale kicks off on October 28. The usual Midweek Madness discounts will continue to happen too, so in other words, there's never a good time to pay the usual asking price for a game.
Epic Games Store is also having a Halloween sale which is already live, because what videogame storefront would miss the opportunity?