It looks like someone leaked the internal e-mail Steam had sent out to their partners about the upcoming Steam Autumn and Holiday Sale 2013. Funny that in red on the e-mail it says “Please DO NOT make the dates of the Sales public”. Whoops! A Reddit user leaked the e-mail and now everyone seems to know all of the details about both sales.
The Steam Autumn sale will run from November 27th to December 3rd. Steam will not accept submissions from game publishers and developers for reduced pricing later than November 20th. The Steam Holiday Sale will begin shortly after starting on December 19th and running through January 2nd.
Of course we do not know what games will be on sale during the two sales, although apparently Counter-Strike: Condition Zero will be offered at $4.99, which is 50% off. It will also be part of an additional Steam promotion (Daily Deal, Flash Sale, or Voting Promotion) that will actually make it 75% off during that promotion.