Queen's Day (Dutch: Koninginnedag) on April 30 (or celebrated on April 29 if the 30th is Sunday) is a national holiday in the Netherlands, Aruba and the Netherlands Antilles. It started as a celebration of the Dutch monarchy. It is mainly known for the 'vrijmarkt' (literally 'freemarket'), the one day in the year that anyone is allowed to sell anything in the streets. The night before is celebrated as 'Koninginnenacht' (queen's night).
The holiday is celebrated on this date because it was the birthday of Queen Juliana (1909-2004). When Queen Beatrix succeeded Juliana on 30 April 1980, she decided to keep the holiday on April 30 as a tribute to her mother. Further the weather on her own birthday, January 31, tends to prohibit the traditional outdoor festivities. 30 April, however, is renowned for having very pleasant weather.
I'm gonna enjoy a day of nice weather and good cold beer: