The first Night of Museums took place in Berlin in 1997, and the concept spread through Europe like wildfire: currently over 120 major European cities (and many minor ones) organise their own museum nights. It's a pretty big deal, with thousands of people heading out to take part and visit museums big or small. If you're into this kind of stuff, keep in mind that Kraków is organising the event a night early, giving you a chance to swing by both cities (they're certainly both worth it).