Gaecheonjeol (Korean: 개천절, hanja: 開天節) is a public holiday in South Korea on 3 October. Also known by the English name National Foundation Day, this holiday celebrates the formation of the first Korean state of Gojoseon in 2333 BC. This date has traditionally been regarded as the date for the founding of the Korean nation.
Gaecheonjeol is also recognized in North Korea, although not as a public holiday, with an annual ceremony at the Mausoleum of Tangun, the founder of Gojoseon.
Gaecheon ("Opening of Heaven") refers to 3 October 2457 BCE, the date when Hwanung (환웅) descended from heaven to live with mankind.
The harvest ceremony was celebrated in the Korean realms; Yeonggo (영고) of Buyeo; Mucheon (무천) of Yemaek; Gyeeum (계음) of Mahan and Byeonhan; Dongmaeng (동맹) of Goguryeo; and Palgwanheoi (팔관회) of Silla.
In 1909, Gaecheonjeol was enacted as a national holiday. At first, the holiday was observed on the third day of the 10th month on the lunar calendar, but it has been fixed on 3 October of the solar Gregorian calendar since 1949.