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.
Hangul
Ju Si-gyeong, the linguist who had coined the term Hangul to replace "Eonmun" or "Vulgar Script" in 1912, established the Korean Language Research Society (later renamed the Hangul Society), which further reformed orthography with "Standardized System of Hangul" in 1933. The principal change was to make the Korean alphabet as morphophonemically practical given the existing letters. A system for transliterating foreign orthographies was published in 1940.