It is, of course, Belgium's National Day. This from 'Flanders News':

'Belgium gained independence in 1830. But why exactly is 21 July our National Holiday? The reason is not linked to 1830, as many think, but to 1831.
In 1830, Belgium had broken away from the Netherlands.

In February 1831, the National Congress adopted a constitution which, at that time, was considered as progressive.

It was on 21 July 1831 that Leopold of Saxe-Coburg swore allegiance to the Belgian constitution in the Sint Jacobs Church on the Coudenberg in Central Brussels.'