The date of Steam winter sale leaked by NeoGAF throughout the day was confirmed by Eurogamer.
Valve rarely confirms the timings of its Steam Sales in advance. However the date of the Steam Winter Sale appears to have leaked.
According to news of Eurogamer Steam winter sales starts from 22nd of December to 2nd of January.
In 2015, the Steam Winter Sale also started on December 22 and ran for a similar period. so this leak looks pretty good.