Wikipedia
Gaecheonjeol
Gaecheonjeol (, 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 legendary 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.