The Steam winter sale is going on now. You already know this.
Slay the Spire is on sale. In January, the game is leaving Early Access and getting a permanent price increase. This is the best time to buy it unless you are going to wait for it to be 75% off the new price.
I’m a guy who waits until everything is 75% off and/or $5 or less, and half the time decides I don’t really want it by the time the price gets that low. And I bought Slay the Spire for full price. And it would have received good value for four times that.