Not sure if it helps, but as a sort of a tip, try to check back every 6 hours by putting an alarm, since is the aproximate time that all the flash sales panels will be changed and it will be unlikely to lose all the deals.

OK, so post no. 1 updated with Back to School Sale (500+ games discounted and Flash Sales) started on Monday, September 3rd, at 14:00 CEST (12:00 UTC) and ending September 10th, 22:00 UTC.

So can we expect a Gog birthday sale on the next week ?

It's rather unlikely. We've just closed quite big sale. I guess we should wait at least to October.

Actually there were large sales one after another in the past. And well, the Back to School one seemed to be rather kind of a placeholder sale (although with great discount on Homeworld :) ).
Perhaps the 10th anniversary celebration delay is related to some Cyberpunk 2077 reveal?

Santa Claus 2077 says "Stay affable" ;)

Oh, look, a weekend sale. ;P
https://www.gog.com/promo/20180914_weekend_sale

There's a whole slew of 'Special' streams next week, it could be related to a new big sale, but I don't know if they would run another one so soon after the Back to School sale:
https://teamup.com/kse3cd10acc46016fc

Yep, October 3rd sounds about right. :p

If they gonna run Halloween Sale this year, then first week of October should be perfect choice for Anniversary Sale.

HunchBluntley noticed here (thanks, pal!) that current Weekend Sale is scheduled September 30, 10pm UTC, which means Sunday instead of Monday as usual. I think that's good point and very strong indicator that we should expect something big on Monday.

I don't know, they may have fixed it. On the sale page I can see countdown "2d, 14h 35m 21s left".

No new Monday Sale. Maybe there will be any kind of sale.

Eeeeeek.
*on tenterhooks counting down till 2pm*

First special stream starts at 1am CEST on Wednesday, so whatever GOG is cooking will likely come out tomorrow afternoon.

As posted here, apparently the Steam sale dates have been leaked, so it might be representative as to when GOG will start their sales.

Halloween sale -> October 29 - 1 November,
Autumn Sale -> November 21 - 27,
Winter Sale ->December 20 - January 3.