Carnival is celebrated in various places around Europe and it's the last weekend/few days before Ash Wednesday (when everyone is supposed to be good). Interestingly though, in the Netherlands, only the Southern region celebrates Carnival and the cities up North don't at all.
The festival was from Saturday to Tuesday and is really something different. It's essentially Halloween for all ages, and you should replace candy with beer. The locals get really into their costumes and must have spent a bunch of money on them. We exchange students all had costumes that we mostly bought at a local costume store. Although I didn't usually partake in the day-drinking it was fun to go downtown and see all of the locals getting rather drunk at 2 in the afternoon (every day of this festival). There were just so many colors and music and bars everywhere. It was definitely a fun time to be in Maastricht that most all of the other kids studying in Utrecht came down for the weekend.
Luckily, we had the entire week off for Carnival so I got to party on the weekend and beginning of the week, then planned a trip to Prague on the Wednesday --> Sunday of that week.