30 April was in our DNA
Since 1949, Amsterdam (and the rest of the Netherlands) celebrated Koninginnedag (Queen's Day) on 30 April, Queen Juliana's birthday. Even when Beatrix came to the throne, although she was born on 31 January, the holiday stayed on this date. Mainly because January doesn't exactly offer the best weather for a street party. For Amsterdammers, 30 April was sacred: a national day of flea markets, boat parties and orange madness.
A secret royal operation in the Jordaan
Speaking of Queen's Day: did you know that Queen Beatrix secretly paid a surprise visit to the Amsterdam Jordaan in 1988? And she did it without the permission of Prime Minister Ruud Lubbers! The monarch had arranged the visit in secret with the mayor at the time, Ed van Thijn. Lubbers was only informed afterwards and was not amused.
The prime minister felt the security risks were too great after the squatters' riots at the investiture in 1980. Beatrix wanted to show precisely that she wasn't afraid and that she embraced Amsterdam. A bit of a royal rebel moment, shall we say.
And then: King's Day on 27 April
With the change of throne in 2013 came a new holiday too: Koningsdag (King's Day), moved to 27 April, Willem-Alexander's actual birthday. Talk about confusing! In 2014 plenty of people, mostly tourists admittedly, were still standing cheerfully in orange gear on 30 April... in an empty street. That was partly because 30 April was listed as a national holiday in practically every travel guide. Luckily Amsterdam quickly worked out that partying three days earlier works just fine too (especially when the sun plays along).

Facts and figures: a whole lot of rubbish
Partying is fun, but it does produce something too: rubbish, mostly. After the first King's Day in 2014, Amsterdam collected around 250,000 kilos of rubbish from the streets. That's roughly twice as much as on a normal day! The city council put out more than 300 extra bins and over 200 cleaners were at work to make the city presentable again the next morning. And every year there's still a race between revellers and cleaners: who gets out on the street first?
And Amsterdam? It just kept on celebrating
Whether it's 30 or 27 April: if there's partying to be done, Amsterdam throws itself in wholeheartedly. The canals fill up with boats again, the streets turn into one big flea market, and orange is and remains the colour of the city.