(From wikipedia.org)

Queen's Day is a national holiday in the Netherlands, the Netherlands Antilles, and Aruba on 30 April or on 29 April if the 30th is a Sunday. Queen's Day celebrates the birthday of the Queen of the Netherlands and is supposed to be a day of national unity and "togetherness" (Dutch: saamhorigheid). The tradition started on 31 August 1885 on the birthday of Princess Wilhelmina, later Queen Wilhelmina. Since 1949, after the ascension of Queen Juliana, Queen's Day is Queen Juliana's birthday on 30 April. Although Queen Beatrix's birthday is on 31 January, she officially celebrates her birthday on 30 April.

Queen's Day is known for its "freemarket" (Dutch: vrijmarkt) all over the country, where everybody is allowed to sell things in the streets. Other activities during Queen's Day are children's games, individual musical performances, and music concerts. The night before Queen's Day is celebrated too in some cities, and this is called Queen's Night (Dutch: Koninginnenacht). The largest celebration of Queen's Day is in Amsterdam and of Queen's Night in The Hague. During the celebrations as reference to the colours of the House of Orange-Nassau, people dress in the colour orange, which is sometimes called "orange craze" (Dutch: oranjegekte).