There are usually six Steam Sales each year: Lunar New Year, Spring, Summer, Halloween, Autumn (for Black Friday), and Winter (for Christmas). They're fairly predictable, but exact dates are only revealed at short notice, which keeps us on our toes.
When is the next Steam Sale?
We just had the Steam Summer Sale 2020, so next up is Halloween.
The Halloween Sale normally kicks off a couple days before Halloween - last year it began on 28 October - and runs until just afterwards. It's a shorter sale than most, typically lasting just a few days.
That's partly to avoid too much overlap with the next Steam sale in the calendar - the Autumn Sale, which takes place in late November to match up to Black Friday sales events.