This week maybe? Or next?

Winter?

Steam Winter Sale starts the 19th of December and GOG usually starts their sale before Steam does. So it will be earlier than the 19th. This week or the next, I won't know. Any discussion should take place here: https://www.gog.com/forum/general/gog_sales_historical_dates_and_forecasts, where you'll also be able to see what the previous dates for the Winter Sale were.

It's been leaked more than two months ago that STEAM's Winter sale will take place between December 19–January 2.

GOG has always launched their sales a little earlier that STEAM's. I think this year's Black Friday sale, was the first time ever that GOG launched after STEAM. Anyway, it's guaranteed that we'll be having a winter sale on GOG, one or two weeks from now.

EDIT: ninjaed

Was that the case last year, too? In my memory it did start pretty late last year, possibly even after the Steam sale had started? Then again, I think there were two kinds of bigger sales, curiously enough, one early in December, the other one late in December and into January. (Both were kind of underwhelming, so I don't expect much this year, but that's a different topic.)

Strangely, the Weekly Sale from last week expires in a few hours, the Weekend Sale has more than two days on the timer still, and there is no newspost about this weeks Weekly Sale.

they start around 15 CEST lately.

https://www.gog.com/forum/general/local_prices_update/post571

Hmm.. today maybe? :p