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
The Korean neutral vowel was "i". The "yin" vowels were "eu, u, eo"; the dots are in the "yin" directions of 'down' and 'left'. The "yang" vowels were "ə, o, a," with the dots in the "yang" directions of 'up' and 'right'. The "Hunmin Jeong-eum Haerye" states that the shapes of the non-dotted letters were chosen to represent the concepts of "yin", "yang", and mediation: Earth, Heaven, and Human. (The letter "ə" is now obsolete except in the Jeju language.)