Copenhagen as well. I would guess it is a phenomenon in many cities.

I think this is a global event. Even back in my hometown of a small EU country they'd do "Night in a museum" event when you could attend museums after midnight.

Same thing in Zürich: https://langenacht-zuerich.ch/en

We have dozen different long night of x events in Berlin. I have only been to the science one, because I don't think any painting can compete with standing next to a particle accelerator.

Same in Wien (Vienna)

same with berlin https://langenachtdermuseen.berlin/en/

incredibly common for big cities