When will be the GOG Christmas/Wintersale?

Probably January

Oddly enough, they usually have it in the summer.

No... that's the Summer Sale... backward northern hemisphere... :/

Not til December, I think we have a Fall sale coming up very soon. Might be the reason we don't have the normal Weekday sale that's normally posted today.

December.

Sometime in Winter. Or so they say.

A week or two before Steam's winter sale is the usual case.

I don't believe you.

The big fall sale comes first (generally in mid-November, lasting for two weeks or close to it); The big winter sale usually begins less than two weeks after the fall sale ends, and lasts for a week and a half to two weeks. In other words, the winter sale is over well before Christmas. Plan accordingly. :)

Up here it's still summer.

We didn't even had Autumn sale... -_-

I don't understand why we have the Fall and Winter sale so close together.

And why don't we have a big Spring sale? or did I miss it (only been buying here for a year)

Sometimes they don't call some of the major seasonal sales after the actual season (usually fall and spring). This year's big spring sale, for example, was the weirdly-named (and underwhelming) Sleepy Sheep Insomnia Sale (and 2015's was also an Insomnia sale).

As to why the late-year seasonal sale are held so close together, someone already mentioned the main reason earlier in the thread: they're trying to avoid getting stepped on by the Steam winter sale.

winter sale already started, its called "discount free winter sale 2016"