King's day (also called Koningsdag in Dutch) is one of the most important national holidays in the Kingdom of the Netherlands. The holiday is celebrated on 27 April (this changes to 26 April if the 27th is a Sunday), since the date marks the birth of King Williem-Alexander. Until 2013, the holiday was known as the Queen's Day, since Queen Beatrix was still on the throne and was not succeeded by her son.
The holiday was initially observed on 31 August 1885, and back then it was called Princess's Day since the holiday celebrated the first birthday of Princess Wilhelmina. The festival is known for its nationwide free market, which means that the Dutch people sell their used items. If you are looking for cheap deals on used Dutch items, the King's day is the date to visit the Netherlands. Be sure to wear an orange shirt, though.