Originally, Belgian National Day was celebrated on 27 September as the commemoration of the Dutch military's final withdrawal from Brussels after several days of fighting in mid-September 1830.
In 1890, the date of the National Day was changed to 21 July to commemorate Leopold I's accession to the throne.
In February 1831, the National Congress adopted a constitution which, at that time, was considered as progressive. On 21 July that year Leopold of Saxe-Cobourg swore allegiance to the new Belgian constitution and became the first King of the Belgians.
The king's vow marked the start of the independent state of Belgium under a constitutional monarchy and parliament. The great powers of the time then recognised the Belgian independence.
Many people, including Belgium's leading politicians, can't remember exactly why 21 July is our National Day.
In 2007, the then Prime Minister Guy Verhofstadt (Flemish liberal) was only able to answer after several minutes of deep reflection. However, he did better than his successor Yves Leterme (Flemish Christian democrat) and Walloon PM Rudy Demotte (Francophone socialist), who could not answer.