Dutch national celebration of the queen's birthday (which is actually on 31 January). 30 April is the birthday of Princess Juliana, the queen's mother. It was kept as the date for queen's day because the middle of winter is bad timing for outdoor festivities... When Beatrix abdicates the date may change.
On queen's day the royal family visits one or two villages, towns or cities in the country, where they are subjected to all the folklore the population of those places can muster: clog making, folk dancing, childrens' choirs, you name it.
The rest of the Dutch have fun on the streets, listening to bands and buying or selling junk at that all time Dutch favourite: the flea market. A popular (but unwise) place to celebrate Koninginnedag, as the day is known in Dutch, is Amsterdam. Here the streets get so crowded that people sometimes get stuck - literally. Beer, sateh, space cake and all kinds of orange food (orange being the national colour) can be found everywhere else as well though. A good place to go (less busy than Amsterdam but still very lively) is Utrecht.