Dangun and nationalism
While Shamanism, Buddhism, and Confucianism were the dominant religious and philosophical movements in Korea before the 20th century, Gosindo (고신도; 古神道) existed as a cult, which had largely died out by the 15th century. In the late 19th and early 20th centuries, with a resurgence in Korean nationalism, the movement was revised in the cults of Daejonggyo (대종교; 大宗敎) and Dangungyo (단군교; 檀君敎). The latter was promoted by Na Cheol (1864-1916), but could not survive the Japanese Colonial Period (1910-1945) (Taejonggyo (1999)/Tangun), since it conflicted with the state Shinto myth of the Japanese emperor's divinity.
Until the end of World War II, Japan promoted the myth that its emperors were desenceded from the goddess Amaterasu. On November 9, 1928--the eve of the deification of Hirohito and the official beginning of his imperial reign--the Donga Ilbo (a widely read Korean daily newspaper) snubbed the official Japanese celebrations in Korea and attempted to reinvigorate Korean nationalism by publishing an account of the Dangun myth (Bix 2001). After independence in 1945, Daejonggyo and Dangungyo were revived, but lacked mass appeal. Nevertheless, campaigns to teach the Dangun myth as historical fact in schools partially succeeded. Since 1988, the national history textbook has explained that Dangun Wanggeom was the ruler's title and that the legend of Dangun reflects historical fact, a move that has since been one-upped in Japan by attempts in 2000 to treat the mythical emperor Jimmu's reign as historical fact in a new textbook [1].
Until 1961, the official South Korean era (for numbering years) was called the Dangi (단기; 檀紀), which began with 2333 BC, the year of the mythical founding of Joseon by Dangun. Daejong-gyo designated October 3rd in the lunar calendar as Gaecheonjeol (개천절; 開天節), or the "Festival of the Opening of Heaven." This day is now a national holiday in the Gregorian calendar, called "National Foundation Day." This is also similar to the situation in Japan, where Jimmu's mythical foundation of that country is celebrated on February 11 of each year [1].
In 1990s, in a move to try to legitimize itself, the North Korean government claimed it had found and excavated the Mausoleum of Dangun.
References
Bix, Herbert P. (1988). Hirohito and the Making of Modern Japan. New York: Perennial.