The Autumn "Black Friday\Exploration" sale is from November 25th 10AM PST til December 1st 10AM PST.
The Holiday sale is from December 22nd 10AM PST til Jan 4th 10AM PST.
From the steamworks developer group....
"In past sale events, we’ve asked for two discounts—a duration discount that runs for the length of the sale, and a feature discount that runs in the event that a game is featured on the front page. This year, to optimize the sale for customers and to allow us to feature and recommend your products in more ways to more customers, we’re asking for just one discount, to run the length of the sale.
We’ll still be highlighting top games on the front page for 24-48 hour spans, but those products will stay at their most competitive discount, before and after being featured. It’s not a major change, but it does make the sale a lot more valuable for customers, and it allows us to build sale tools that highlight and recommend products all sale long, instead of just during front page features."
Source: Neogaf