It's a function of the climate, if you excercise your right to four consecutive weeks, your employer has the right to pick the dates, as long as they are sometime between June and August, since that's when our summer is.

For this reason we have the "industrial vacation" in July when all heavy industry shuts down, since it's easier if everyone and the entire supply chain shut down at the same time.

In the UK these are often called "trade holidays" and used to be set on a city by city basis - this seems to have died out now that we have relatively little heavy industry left.