They party like no one else
Some of the nicest street and cultural celebrations I have ever been to took place in Holland. They celebrate Carnival with much enthusiasm and joy as a Brazilian or a Venetian. But there are two parties that cause lots of uproar and cannot be missed, which you should go at least once in a lifetime: Koninsdag (King's Day) and the Gay Parade.
Both are conceptually very distinct, albeit having many similarities: they happen on the streets, attract people from all over the country, are supported by local administrations, and gather all types of public. It is even more interesting to note that while one party is a national holiday that celebrates King’s Day, the other has a social aspect because it honors tolerance torwards different sexual choices and it defends the rights of gays.
What enables these two amazing and exciting parties, albeit being so distinct, to attract such a large number of people at the same time is the fact that the Dutch have a tolerant spirit and behavior like no other nation (link with tolerance article).