Why orange? The Dutch royal family is called the House of Orange. Orange has been their colour for over 400 years. So on the King's birthday, people show their celebration by wearing orange shirts, dresses, hats, hair, even painting their faces orange.
One special King's Day tradition is the free market - in Dutch, vrijmarkt. For one day only, anyone is allowed to sell things on the street without paying. Children spread blankets in front of their houses and sell old toys, books and homemade lemonade. It is a bit like a giant garage sale across the whole country.
In Amsterdam, the canals fill up with boats. Each boat is decked out in orange flags and orange flowers, with music playing. People dance on the boats and along the bridges. Helicopters in the sky show that from above, the whole city looks like a fizzing orange drink.
The Dutch national football team also wears orange. Their fans are nicknamed the 'Oranje legioen' (the Orange Legion). When the football team plays, you can sometimes see whole stadiums turn orange too - a smaller version of what happens on King's Day.