Hangul Day
Before the creation of Hangul, people in Korea (known as Joseon at the time) primarily wrote using Classical Chinese alongside native phonetic writing systems that predate Hangul by hundreds of years, including idu, hyangchal, gugyeol, and gakpil. However, due to the fundamental differences between the Korean and Chinese languages, and the large number of characters needed to be learned, there was much difficulty in learning how to write using Chinese characters for the lower classes, who often didn't have the privilege of education. To assuage this problem, King Sejong created the unique alphabet known as Hangul to promote literacy among the common people.
Yeomillak
Yeominrak, which means "Enjoyment with the People", was created based on Korean court music that the king shared to his people to be enjoyed together. This song was created during his 29th year of reign (1447) and actually had lyrics from the Yongbi eocheonga, but only the melodic component is existent today. It is usually played on Hangul Day in South Korea to celebrate the promulgation of Hangul by King Sejong, and also in religious services for the Ancestral Shrine of the Royal family, honoring the Royal Court, and for the Civil and Military authorities.