Even though Valve usually keeps the dates of Steam sales close to the chest until almost the time they go live, the details usually end up leaking anyway. This time, though, Valve is getting ahead of the curve. It confirmed when the next three sales will take place.
As spotted by Eurogamer, the Steamworks Documentation page notes that the Steam Halloween sale is this weekend and it runs between October 28th and November 1st. The autumn sale will take place from November 24th-30th. As for the Steam winter sale, one of the platform's two biggest sales of the year alongside the summer edition, you'll be able to score discounts on a ton of games between December 22nd and January 5th.
It's not a bad idea to reveal the dates well in advance. Steam connoisseurs know the sales are coming anyway and the dates are less important than announcing which games will be included beforehand. If a player knows that a game they want to buy will likely get a hefty discount in a couple of months, they'll be less inclined to buy it now. In any case, Steam users now know when they'll be able to stockpile games for their Steam Deck.