Valve can't keep sales secret. Every year, the big dates leak well in advance, whether whispered by devs or blasted by PayPal newsletters. This year, Valve are apparently content to say them in public where anyone might here. Hey, it's just a sale, why the secrecy? So yup, we now definitely know dates for the Steam Halloween Sale, the Steam Autumn Sale, and the Steam Winter Sale.
Your Steam sale dates for the rest of the year, as confirmed by Valve's Steamworks Documentation, are:
Steam Halloween Sale: Oct 28 - Nov 1, 2021
Steam Autumn Sale: Nov 24 - 30, 2021
Steam Winter Sale: Dec 22, 2021 - Jan 5, 2022
Valve don't have much to say about the autumn or winter sales yet, but do note some of the festivities planned around Halloween. They plan to highlight Halloweeny in-game events, additions, bonuses, and such on the hub sale page as well as in game feeds. But seemingly no fancy minigames or things that devs can join in with. If any of that guff will return for Christmas, they're not saying yet.
Perhaps you might want to prepare by taking 30 minutes to go clear out your Steam Inventory of surplus trading cards, hats, and other gubbins that might fetch you a few pennies.
I no longer buy great sackfuls of games in Steam's seasonal sales, though I think that's largely because I still haven't played many of those panic purchases. I have so very many video games now. But I do still appreciate big discounts in the big sales to pick up games that slipped out of mind. Even I often use the RPS Advent Calendar as a guide to scoop up things I missed in the Steam Winter Sale.