Constitution Day (제헌절) in South Korea is on July 17, the day that the Korean Constitution was proclaimed in 1948. The date was deliberately chosen to match the founding date of July 17 of the Joseon Dynasty.

Although Korea was liberated from Japanese rule at the end of World War II on August 15, 1945, the peninsula was caught in the middle of the Cold War between Russia and the United States and it was only in 1948 when a democratic election for National Assembly members was held separately in South Korea. The elected assembly members set upon creating a constitution, and decided upon a presidential and unicameral system. The constitution was formally adopted and promulgated by President Syngman Rhee on July 17, 1948.

Constitution Day was proclaimed a national holiday on October 1, 1949, with the creation of the National Holiday Law. A commemorative ceremony is held with the President, Chairman of the National Assembly, Chief Justice of the Supreme Court and the original constitutional assembly members in attendance, and citizens hang the national flag in commemoration. Special activities such as marathons are often held.
Since 2008, Constitution Day in South Korea is no longer a “no work” public holiday, following the restructure of laws regarding the public sector with a 40-hour work week. It is still a national holiday for commemoration.

Source: wikipedia http://en.wikipedia.org/wiki/Constitution_Day_(South_Korea)