=== 2020 ===
Harvest Sale - - - - - 24 - 31 Aug
Summer Sale - - - - - 27 May - 15 Jun
Spring Sale - - - - - 16-30 Mar
Lunar Sale - - - - - 24-31 Jan

Farmers rule!

Yee haw!
So CervelloYM was right – there cannot be "Back to School Sale" without coming back to schools ;) Let's focus on harvesting crops for the time being.

Thanks, OP has been updated. With 1400 games on sale the event definitely deserves its place on List of Big Sales.

So, the summer sale is during spring and the harvest sale is before autumn. Any more sales this year?

The anniversary of gog is soon, in a couple of weeks i think, and they will do a little event, maybe even with a gift game.

Also in one month (end october/principles of november) or so will arrive the Halloween sales.

Finally before the winter sales that are three weeks there will be at the end of november/begin of december the black friday for a couple of days, at least the last year was.

Are there any vague signs that the GOG birthday sale will start this Friday? I saw some rumors about it on the forum (for example, here).

Maybe someone has already found something that is not available to the eyes of regular (not so tech-savvy) users? (:

I'd be really surprised if sale started this week.
We've got Weekly and Midweek sales ongoing.

Argamasa was right. We will get three or four more sales this year. :D

1. The 12th Anniversary Sale of gog is coming soon (in a couple of hours i think?):
http://www.gog.com/promo/2020_gog_12_anniversary_sale

2. Halloween Sale (May be)

3. Black Friday Sale/ Fall Sale

4. Winter Sale

Thanks, I found a link of the anniversary sale from mount & blade ii butterlord game page.
https://www.gog.com/game/mount_blade_ii_bannerlord

http://www.gog.com/promo/2020_gog_12_anniversary_sale
I won't be surprised if sale started today/tomorrow, Oct 2. :D

That's interesting. However I can't see the link - it's somewhere in metadata or can be found directly from webpage?

True. But I think GOG tries to avoid overlapping small and big sales recently - it causes that at some point some games disappear from big sale list, which causes confusion.

They don't seem to be trying to avoid it this year. Of the major sales for 2020 in the first post, the Lunar sale started on a Friday and the Summer Sale started on a Wednesday.

Did they shorten or skip the Monday weekly sales in those weeks?

You're right and I'm wrong :) I've checked the thread with all sales and it's as you suggested - these two started when other small sales was up and running. I don't know where my opinion come from, but it's simply wrong.

It's not wrong, it's just not fully up to date. Until some point in 2019, what you said was true, then GOG (occasionally) changed their mode of operation.

Anyhow, this discussion is beside the point, since this is not a big sale (only 200+ games on sale). It's just a longer weekend sale. I wouldn't bother listing it.

Gog 12th anniversary ended... next station Halloween sale!

I guess as in 2017, from 26 Oct to 2 Nov, in only two weeks, it's soon but we will see...

I'd like Sacred saga with a nice discount.

The site's front page was quite broken for the last day. Historically, gog tends to tinker with the code for the frontpage quite a lot right before upcoming major sales when they introduce new effects. (E.g. the color-coding from the last couple, or the RPG "experience points for free games" thing from the Monstrous sales from 2016.) That means that a major sale in the next two weeks becomes very likely, with my personal prediction being that it'll be accompanied by a new frontpage gimmick. (If it's a Halloween sale, then they were particularly early with their front-end changes; could also be an "autumn sale" starting as early as Monday?)