Queen's day (Koninginnedag), used to be a Dutch national holiday celebrated on April 30th (Queen Juliana's birthday).
When Princess Beatrix abdicated the throne in 2013 after 33 years of ruling, the party and celebration was moved to King Willem-Alexander’s birthday on April 27. It is now called King's Day.
Honouring a monarch
The tradition of honouring a monarch’s birthday with a national celebration began with Queen Wilhelmina in 1885 as Princess’s Day. Held on August 31, once she became Queen, the name was changed to Queen’s Day.
YouTube video by Wolfgang Huerst
Image source: http://www.beweegbewust.info
Queen Juliana was the next royal to have her birthday nationally celebrated in 1948, which relative to weather patterns in the Netherlands, conveniently fell on April 30.
Queen Beatrix wasn't so lucky in terms of birthdays suited for outdoor celebrations, with hers occurring on January 31. In a display of humility, she decided to keep the holiday on April 30 in honour of her mother.