It started from the email which was sent to the Steam community-group (unofficial) on the Russian social network, VK.
Here is a screenshot that was uploaded to Imgur, of the email from Valve:

I still don’t know why the person who leaked this decided to ignore the red text, where Valve specifically asked people to do not make the date of the Sales public.
The guy said that this information is ‘highly restricted’, so, because of the restriction, I decided to ignore the leak as I did not want to be responsible for increasing publicity for the leak any more than it had been already.
However 6 to 7 hours later I found the same post on a forum/site related to Valve news, ValveTimes.com (not to be confused with ValveTime.net), who claimed it was an exclusive post.
Then, an hour after that, the leak was also posted on Reddit.
I can’t tell who leaked this information and what was the game he was developing, but right now I’m 100% sure that Valve are investigating this leak and the guy who did this will be ‘punished’.
We hope that this leak will not very harm Valve or the publishers and developers involved in the sale.
Disclaimer: LambdaGeneration respects the integrity and efforts of both Valve and the community. We do not believe in posting leaks of any content directly. This article is simply a reference/report of content which has already been widespread across the internet and/or other mediums.
Article Sources
Thanks to El Calhau for the initial tip on this finding
ValveTimes Post on the Leak
Thread on Reddit
Around the International Community
Steam VK – Анонимный русский разработчик слил даты распродаж в Steam
HL Portal – Datum für Herbst- und Holiday-Sale geleakt
How would it harm them?
We don’t know Steam do sales already?
You, me and many more people know sort of when will the sales appear, but the fact is we never really know when they’re coming. I mean, there are two main issues here:
First, the people who don’t really pay attention to when big sales are coming to steam may most likely know already, which makes them wait to buy that game they want on steam now a few more weeks just to get the discount;
Second, because of the many new developers coming to steam trough steam greenlight, Valve has the problem of not being able to trust them just because they may not care or whatever the hell theirs reasons are, that puts Valve in a bad position, and the guys who really get mad are the big publishers.
This is going to become an even bigger problem over time because valve as to email every single little indie dev that got lucky enough to get their game to steam through steam greenlight… It’s not going to be an easy issue to resolve…
The person who leaked this is an ass…
I hope they catch him
Welp… *gets my wallet ready*… This is going to be a looong christmas.
It’s gonna be a short one for your wallet