Here' some info
This is a very old tradition, which goes back to the 14th century, when the actual Belgium was part of a bigger realm, under the power of the dukes of Burgundy.
In 1356, the reigning monarch came to Brabant to swear the oath on the charter of the 'Joyeuse Entrée' which recognised the rights and privileges of his subjects.
This text became one of the sources of the Belgian constitution in 1831.
Untill 1792 all reigning princes (Spaniards, and later Austrians, who dominated the territory which is now Belgium) sweared the oath on this text.
All kings of the Belgians, are being welcomed in an official way in the capitals of the Belgian provinces and the most important cities, since 1831.
The tradition of a crownprincely couple also making these visits has been introduced in 1900, by prince
Albert and princess Elisabeth.