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.
Hunminjeongeum
Hunminjeongeum (lit. "The Correct/Proper Sounds for the Instruction of the People") is a document describing an entirely new and native script for the Korean language. The script was initially named after the publication, but later came to be known as hangul. It was created so that the common people illiterate in hanja could accurately and easily read and write the Korean language. It was announced in Volume 102 of the "Annals of King Sejong", and its formal supposed publication date, October 9, 1446, is now Hangul Day in South Korea. The Annals place its invention to the 25th year of Sejong's reign, corresponding to 1443–1444.