Hmm .

The timer on the weekly sale say " 1 days 3 hours left " while on the gae page it shows " 12 hours left "

Yeah, it looks like the Spring Sale will start on Wednesday 20 March and end on 4 April.

You can't even imagine how much I can't wait for SpellForce: Conquest of Eo and the Strategic Command series to go on sale.

I wonder what BYOB offers and free games will we get this time .

Spring Sale added to OP.

Looks like none, at least at the moment ¯\_(ツ)_/¯

Free Book of Demons.
https://www.gog.com/en/game/book_of_demons

And so spring sale is over, next up summer sale!
My prediction is that summer sale will start on Monday June 10th and end on Monday July 1st.

This is my prediction simply because it matches what was done last year: a three week sale with start on the Monday two and a half weeks ahead of the Steam summer sale in order to get ahead, but with the last weekend of sale overlapping with Steam and the sale ending on Monday after the weekend. I admit that GOG could choose to start the sale on Wednesday the 12th (and possibly ending it on Wednesday July 3rd) since it seems that they have started more sales on Wednesdays lately, but I feel relatively confident about the bulk of my prediction.

Is there any relatively big sales that usually comes between spring and summer sale?

I checked Ciemnogrodzianin's table, and it seems that there are no big sales between the spring and summer sales.

Hey, that's interesting. It looks, indeed, like a longest period without any sale in the last years.
We have to wait till June (end of May perhaps) for Summer Sale. No other hope :)

You forgot about May 4th Starwars/Disney sale. :)

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Oh, but I do not think it may be considered a full-scale sale – they're never as big as seasonal sales tracked here.

And the Warhammer Skulls sale :)