Any indications when it will begin?

Yes, indications are it will begin very soon; today or possibly tomorrow, Friday at the latest.
Have a look at when this sale ends: https://www.gog.com/promo/20201211_weekend_sale

Please let there be unlockable free games based on the amount of purchases you made like they used to do back in the day. i remember getting so games like sunless sea and dreamfall chapters with those special events.

It mustn't be a special. Buy 10 games get 1 of your choice free. (Sum-total / 10) - x% = price range of free game to select from. This would incentivize people to purchase outside of sales. If GoG were adding the amount as store credit it is very well possible to get numerous games for free instead one very expensive one.

I think the Christmas/Winter sale will begin today, 16 December 2020, at UTC 2pm.

Weekend sale ends in a couple of hours. It is the only sale left, no weekly sale this time, so my bet is that the Christmas Sale will be today.

Christmas sale won't start before everyone has bought a copy of Cyberpunk 2077. ;P

Yesterday there was a tweet from the publisher of Prison Architect that the game will be gifted today on gog starting 15 CET. The tweet seems to be removed, but I‘ve read it yesterday after someone posted it in following thread:

https://www.gog.com/forum/general/gog_sales_historical_dates_and_forecasts

12 minutes to go :)