Koningsdag (Dutch pronunciation: [ˈkoːnɪŋzdɑx] ) or King's Day is a national holiday in the Kingdom of the Netherlands. Celebrated on 27 April (26 April if the 27th is a Sunday), the date marks the birth of King Willem-Alexander. When the Dutch monarch is female, the holiday is known as Koninginnedag (Dutch pronunciation: [ˌkoːnɪŋˈɪnədɑx] ) or Queen's Day and, under Queen Beatrix until 2013, was celebrated on 30 April. She prolonged the tradition that was born under her mother’s reign: Queen Juliana’s (Queen of The Netherlands from 1948 till 1980) birthday was on April 30th.

The holiday was initially observed on 31 August 1885 as Prinsessedag or Princess's Day, the fifth birthday of Princess Wilhelmina, then heir presumptive to the Dutch throne. On her accession in November 1890 the holiday acquired the name Koninginnedag, first celebrated on 31 August 1891. In September 1948, Wilhelmina's daughter Juliana ascended to the throne and the holiday was moved to her birthday, 30 April. The holiday was celebrated on this date from 1949.

Juliana's daughter, Beatrix, retained the celebration on 30 April upon her ascent to the throne in 1980, because her birthday was on 31 January, the middle of winter. Beatrix did alter her mother's custom of receiving a floral parade at Soestdijk Palace, instead choosing to visit different Dutch towns each year and join in the festivities with her children.

In 2009, the Queen was celebrating Queen's Day in the city of Apeldoorn when a confused man drove into a crowd of people and crashed into a monument: seven people in the crowd were killed, as was the driver.

Queen Beatrix abdicated on Koninginnedag 2013, and her son, Willem-Alexander, ascended the throne (the first king since the observance of the national holiday). As a result, the holiday became known as Koningsdag from 2014 on, and the celebration was moved three days earlier to 27 April, Willem-Alexander's actual birthday.

Koningsdag is known for its nationwide vrijmarkt ('free market'), at which the Dutch sell their used items. It is also an opportunity for "orange madness" or oranjegekte, a kind of frenzy named for the national colour.

During the reign of Queen Wilhelmina, Queen's Day was celebrated on August 31st, her birthday until 1948. After Juliana's coronation, from 1949 onwards, the date was moved to her date of birth, April 30th. When Queen Beatrix ascended to the throne in 1980 she chose to not move this date in honor her mother. An additional practical consideration for her was that on her own birthday, the 31st of January the weather was not deemed suitable enough for a large-scale outdoor event, unlike the end of April. Between 1949 and 1980, because of religious observation, if the 30th of April was a Sunday, Queen's Day would move up by one day to May first. However, since 1980 this was turned around to instead take place a day earlier, so that the celebrations would be held on the Saturday preceding the 30th. This was the case in 1989 for the first time.

After the coronation of King Willem-Alexander, the date changed to that of his date of birth, April 27, from 2014 onwards, if not on a Sunday. If that occurred, which was true for 2014 itself as well, King's Day will again be held on Saturdays preceding the 27th.

Faced with an unpopular monarchy, in the 1880s the liberals in Dutch government sought a means of promoting national unity. King William III was disliked, but his four-year-old daughter Princess Wilhelmina was not. A holiday honouring King William had been intermittently held on his birthday, and J. W. R. Gerlach, editor of the newspaper Utrechts Provinciaal en Stedelijk Dagblad, proposed that the princess's birthday be observed as an opportunity for patriotic celebration and national reconciliation. Prinsessedag or Princess's Day was first celebrated in the Netherlands on 31 August 1885, Wilhelmina's fifth birthday. The young princess was paraded through the streets, waving to the crowds. The first observance occurred only in Utrecht, but other municipalities quickly began to observe it, organizing activities for children. Further processions were held in the following years, and when Wilhelmina inherited the throne in 1890, Prinsessedag was renamed Koninginnedag, or Queen's Day. By then almost every Dutch town and city was marking the holiday.

The celebration proved popular, and when the Queen came of age in 1898, her inauguration was postponed six days to 6 September so as not to interfere with Koninginnedag. The annual holiday fell on the final day of school summer vacation, which made it popular among schoolchildren. It is uncertain how much Wilhelmina enjoyed the festivities; although writer Mike Peek, in a 2011 magazine article about Koninginnedag, suggests she was enthusiastic, there is a story of Wilhelmina, after a tired return from one of these birthday processions, making her doll bow until the toy's hair was dishevelled, and telling it, "Now you shall sit in a carriage and bow until your back aches, and see how much you like being a Queen!"

Koninginnedag 1902 not only honoured the Queen's birthday, but was celebrated with increased enthusiasm as it marked her recovery from serious illness. Wilhelmina rarely attended Koninginnedag festivities after reaching adulthood. She attended ceremonies for her silver jubilee in 1923, which included massive festivities in Amsterdam and The Hague, despite the Queen's request that large sums not be spent because economic conditions at the time were difficult. To ensure that even the poorer parts of the city were included, bands played simultaneously at 28 locations across The Hague. Wilhelmina made further exceptions for such events as her fiftieth birthday in 1930. During the German occupation of the Netherlands during World War II, Koninginnedag celebrations were banned, and members of the Orange Committees, which organize the holiday events, destroyed their records for fear of German reprisals.

Another summertime birthday celebration in the Netherlands was that of Wilhelmina's mother, Queen-Regent Emma, who after Wilhelmina attained adulthood generally spent her own birthday, 2 August, at Soestdijk Palace in Baarn. Until her death in 1934, Emma received an annual floral tribute from the townsfolk on her birthday. In 1937 Wilhelmina's daughter and heiress, Princess Juliana, took up residence at Soestdijk Palace following her marriage, and the townsfolk made their floral presentation to her, moving the date to Juliana's birthday, 30 April. In September 1948 Juliana ascended to the Dutch throne and from 1949 onwards Koninginnedag was on her birthday. The change in date attracted immediate approval from Dutch children, who gained an extra day of holiday. The first observance of the holiday on the new date included a huge circus at the Amsterdam Olympic Stadium—one not attended by the royal family, who remained at Soestdijk Palace. Queen Juliana retained the floral tribute, staying each year on Koninginnedag at Soestdijk Palace to receive it. The parade became televised in the 1950s, and Koninginnedag increasingly became a national holiday, with workers given the day off. Juliana had a reputation as a "queen of the people", and according to Peek, "it felt as if she invited her subjects to the royal home".

In early 1966 Juliana's eldest daughter, Princess Beatrix, married Klaus-Georg von Amsberg. The marriage was controversial because the new Prince Claus (as he was dubbed) was a German, and Claus himself had served in the German Army during the war. Anti-German riots in Amsterdam marred the wedding day and the following observances of Koninginnedag. Fearing further demonstrations on the holiday, government officials decided to open Amsterdam city centre to the vrijmarkt ("free market") that had long been held on Koninginnedag in the outskirts of town, principally for children. The vrijmarkt occupied the space where demonstrations might have been held, and began a new custom.

When Queen Beatrix succeeded her mother Juliana on the latter's abdication on 30 April 1980, the new queen decided to keep the holiday on 30 April as a tribute to her mother. (If 30 April fell on a Sunday, Koninginnedag was observed the previous day—this occurred most recently in 2006.) The reason was practical as well—Beatrix's actual birthday on 31 January would have been less conducive to the traditional outdoor activities. Rather than remaining at the palace and letting the Dutch people come to her, Beatrix instead usually visited two towns each year for Koninginnedag celebrations. Local crafts and customs were demonstrated for the royal family, who had the opportunity to join in.

Koninginnedag celebrations have sometimes been affected or disrupted. In 1988 three British servicemen stationed in Germany who were in the Netherlands for Koninginnedag were killed in Irish Republican Army attacks. In 1996 the celebrations in Rotterdam were dampened by an alcohol ban, put in place following riots earlier in the week after local football club Feyenoord won the Dutch league championship. The Queen's scheduled 2001 visits to Hoogeveen and Meppel were postponed for one year owing to an outbreak of foot-and-mouth disease.

On 30 April 2009, Beatrix and other members of the royal family were at the town of Apeldoorn when a 38-year-old man, Karst Tates, drove his Suzuki Swift automobile into the crowd, narrowly missing the open-top bus the royal family members were riding on. Seven people were killed and further celebrations were cancelled. Tates died of injuries sustained in the attack soon afterwards and his exact motives remain unclear, though it appears his target was the royal family. The incident provoked questions about whether the royal family should continue to participate in the celebrations. However, Beatrix indicated that the tragedy would not stop her from meeting her people. In 2010, Beatrix and her family visited Wemeldinge and Middelburg, in Zeeland province. There were no incidents, and afterwards, the Queen thanked Zeeland for giving Koninginnedag back to her family, and to her country.