The large Black Friday banner said "BLACK FRIDAY November 26th - November 29th", and in sales like this, communicating it this way very commonly means that the end date is an "until end of day" date. Given that the 29th is the so-called Cyber Monday, it's even easier to assume that this is what the banner means.

Yup exactly, the banner said thru the 29th and now the games in my cart/wishlist are no longer on sale. I would have purchased last night if I knew the sale was ending. Why wouldn't GOG just let the sale go through Monday? Strange decision

A small correction, to keep this accurate: the banner didn't say "thru the 29th", it just contained the timeframe, "November 26th - November 29th" (see the attachment) - which is easy to assume as "thru the 29th", yeah, I agree.

In the emails received from gog (promoting this sale) it says explicitly "The offer ends on 29th November 2021, at 2 PM UTC", so it was planned like that all along, and even communicated in the emails in that fashion.

Of course, this doesn't change the fact that, when just seeing the banner and entering the game listing from there, it was not far-fetched to assume the sale was going to be thru the 29th :), and it's understandable someone did assume that.

I too assumed it would last until the end of the 29th.
Oh well, saved even more money this way.