Munich has a series of "Long Nights" scattered about the year.
Long Night of Architecture: January (every other year)
Long Night of Music: May
Long Night of Clubs: May
Long Night of Culture and Shopping: September
Long Night of Museums: October
And there might be others that I don't know about...
These are usually nights centered around culture and discovering Munich. For example, on the Long Night of Museums, the museums in Munich are open until 2AM, and one flat fee will get you into all the museums participating in the event. Shuttle buses run between the museums, and most of the museums have bars, possibly even music or special entertainment, and certainly special exhibits and lectures.
Similarly, the Long Night of Shopping is a special event where the stores stay open to shoppers until midnight, and street performers create music and put on shows throughout the central pedestrian area of Munich.
On the Long Night of Music, something like 400 concerts take place across the city, and revelers can wander from one concert venue to another, taking in all kinds of musical performances.
"The Long Nights" are events that were originally devised in Germany; the first Long Night of Museums took place in Berlin in 1997 to introduce potential new patrons to the museum culture in Berlin. It must have been a huge hit, because most of the major cities in Germany now host such events, and the concept has spread to several other countries around the world. I think it's natural that Germans embrace the "Long Nights" because in general German public institutions (and stores, restaurants, beer gardens, etc.) close down very early, so it feels festive and exciting to have a few nights per year when these places throw open their doors to the public, and turn the streets into a celebration of culture.
Although the Long Nights were devised here in Germany, I understand that the idea can be traced to St. Petersburg's White Nights Festival, which is one of the largest cultural events in the world (anyone want to go? This could be a seriously awesome long weekend trip next summer...). Anyway, if you don't mind crowds, it's a fun way to get to know Munich!