Roskilde Festival takes place each summer (typically late June to early July) on the fields south of Roskilde, and for that week the population of the town effectively doubles. It draws around 130,000 visitors across eight days and attracts major international headliners alongside a programme of world music, electronic, and emerging artists that has historically given it more curatorial ambition than most festivals of its size.
The site is a city in itself during the festival — with camping for most attendees, multiple stages, and a sprawling infrastructure. The non-profit structure, in which all proceeds go to humanitarian and cultural causes, gives the event an unusual atmosphere: less corporate, more community-oriented than comparable events. If you're in Copenhagen in late June and can get tickets (they sell fast), it's worth the short train journey.