According to steamdb, the Halloween sale will take place tomorrow, October 29th, and will last until November 1st.
 
Information about 2 other sales has been leaked and might change, but it should be around same dates:
 
Autumn sale will possibly run from November 21st to November 27th, and Winter sale - from December 20th to January 3rd.
 
During Halloween discount we might pick up some spooky games, while Autumn one will bring more titles.
 
But the largest sale will be the Winter one, since it combine 2 big events: Christmas and New Year.
 
During this time it might also include special events like steam awards (as it was previously), new badges, trading cards and other surprises.