The story
Korean was written in Chinese characters, which took years to learn and kept literacy inside a small class. King Sejong had an alphabet made in which the consonant shapes copy the mouth that makes them and the vowels are built from three strokes.
The preface says that a wise man can learn it in a morning and a fool in ten days. The scholars of the court opposed it for exactly that reason, and it took four centuries to become the ordinary script.
It is one of very few national holidays anywhere given to a writing system rather than to a person or a battle.