Although not well documented, it used to be a thing where a large celebration would be held for couples that get engaged in noble families (usually high ranking ones). It is often exaggerated in novels and other media the extent of the celebration, but they still happened in various cultures around the world. I guess since the characters are still young the author chose an engagement ceremony instead of straight marriage due to the fact that they are still underage and it would be cold hearted to send the mc away right after their wedding night.
oh that makes so much sense in the noble social setting thanks for sharing
why are they engaged when they in a wedding ceremony?