Koningsdag takes place on April 27, 2018. Koningsdag or King's Day is a national holiday in the Kingdom of the Netherlands. Celebrated on 27 April (the 26th if the 27th falls on a Sunday), the date marks the birth of King Willem-Alexander.
From 1890 to 2013, the day was known as Koninginnedag or Queen's Day. The holiday was first observed on 31 August 1885 as Prinsessedag or Princess's Day, the fifth birthday anniversary of Princess Wilhelmina, heiress to the Dutch throne. On her accession, the holiday acquired the name, Koninginnedag. (With material from: Wikipedia)