The information is unofficial, but very plausible.
Information about the dates of the seasonal sales, which Steam will arrange in the near future, has appeared on the Internet. So, the autumn sale is scheduled for November 21-27, and the winter sale will take place from December 20 to January 3.
Although official information will have to wait, these dates look plausible when compared to last season's sales dates: November 22-28 and December 21-January 4, respectively.
In addition, the next Steam sale is coming soon: it is timed to coincide with Halloween and will be held, reportedly from October 29 to November 1.