According to a leak by PayPal, the fated date should be December 22. Last year's Steam Winter Sale began at 1PM EST, so it makes sense to assume the same to be true for this year.
Obviously, it is way too early to make any guess about which games will be discounted in this sale and for how much. That said, you can take a detailed look at last year's edition (which ran over the course of a full week) here to get a glimpse at what might be coming.
As always, stay tuned on Wccftech and we'll let you know what kind of deals are available once they are live on Steam. In the meantime, which game are you looking to snatch as soon as you see it discounted? Tell us in the comments!