It looks like the dates for the Steam Winter Sale may have been leaked- according to user who posts on NeoGAF, who has posted what may be a screenshot taken from a developer-only Steam group, the Winter Sale begins on December 22, and will go all the way through January 2.
The screenshot gains credibility because this sounds in line with previous Steam Winter sales, and also because it has the correct dates for the Steam Autumn Sale… albeit, that sale just concluded some time ago, so actually posting a screen with the right dates for something that already happened isn’t that compelling in the way of evidence.
That said, this is not the first screenshot we have received that leaks this date, and it likely will not be the last. It is now increasingly likely that December 22 is when the Steam Winter Sale will begin- but I guess we will know for sure in a few weeks.