Via its official Steamworks Documentation page we have the dates for the Steam Halloween, autumn and winter sales:
Steam Halloween sale: 28th Oct to 1st Nov 2021 Steam autumn sale: 24th Nov to 30th Nov 2021 Steam winter sale: 22nd Dec 2021 to 5th Jan 2022
These Steam sale dates usually leak, so it’s nice to see Valve just go ahead and confirm its plans well ahead of time. Any games you hope will be part of the sale so you can buy them and never play them?