"So, developers are going to miss out their (northern) summer holidays"
Not every country has a bunch of holidays during that time. Australia, the Land of the Long Weekend, has a holiday period roughly from shortly before Christmas eve to shortly after New Years, which is summer for us, and obviously not (northern) summer. This is not an official holiday period, other than the usual Christmas and New Years holidays, the rest is simply "Meh, we are closed for a couple of weeks" for the majority of companies / government departments. Or the silly season as it is known. I suspect a lot of other countries also close down most things during that period. We don't have any lengthy mid year holiday seasons though, officially or unofficially.
Other than Easter, the rest of our holidays tend to be moved to the nearest Monday or (not so often) Friday, hence Land of the Long Weekend.
School kids get a longer break at the beginning / start of the year, and if I recall (and it's not changed since I went to school) two shorter breaks during the year of two weeks each I think. Yes, I live within earshot of three schools, you'd think I would have noticed the differences in noise levels during my twelve year stay here. I've not really paid much attention.
I understand Europe has a lengthy period during August, where people bitch about all the tourists coming to their locality, so go to some other European holiday destination to get away from them, and go annoy the locals there.
I may be wrong, but I think USA has their famous school summer holidays, that a lot of movies are made about, which sounds like a lengthy mid year break. The movies are the only reason I know about this. I don't put a lot of stock in the accuracy of anything coming out of Hollywood though.
Other countries, and there are a lot of those, make up their own minds about their holiday periods. May or may not match up with USA, Europe, or Australia, purely coincidentally, or to align with international business needs.
So, some developers are going to miss out on their (northern) summer holidays, and some, even in the north, wont have holidays during that time to miss.