Gaecheonjeol (開天節), National Foundation Day

"Gaecheonjeol," or National Foundation Day, is a national holiday held on October 3 to commemorate the foundation of Gojoseon (古朝鮮), the first Korean kingdom, by Dangun (檀君). Foundation Day in 2018, which is year 4351 according to the Dangi calendar, celebrates the nation's 4,350th anniversary. 
National Foundation Day is a national commemorative day related to Dangun, the mythological founder of the first Korean nation, and thus symbolizes the birth of the Korean race. When the Korean national commemorative day was designated for the purpose of commemorating the nation’s foundation by Dangun, the day was named "Gaecheonjeol," which literally means "opening sky day," rather than "Geonguk Ginyeomil" (建國記念日), which means "the day for celebrating the foundation of the nation" in Korean. There is a tradition among the Korean people to view the foundation of Gojoseon by Dangun as the beginning of Korean history. The roots of such awareness date back to the time even before the writing of Samguk yusa (三國遺事, Memorabilia of the Three Kingdoms)". During the Joseon dynasty and the Japanese colonial era, the idea of the Korean race being the descendants of Dangun became popular, thus providing the impetus for the Korean independence movement. The Korean folk belief called Daejonggyo (大倧敎) even raised the belief to a religious level and included "Gaecheonjeol" as part of the religious ceremony.
Following the establishment of the Provisional Government of the Republic of Korea in Shanghai in 1919, the newly formed government held a commemorative event on October 3 of the lunar calendar under the name "Geonguk Giwonjeol" (建國紀元節)), organized by the State Council. In Korea, too, intellectuals also held a commemorative event, and by the mid-1920s the name "Geonguk Giwonjeol" had become generalized as "Gaecheonjeol." After the country’s liberation from Japan and the establishment of an official Korean government, National Foundation Day was designated as a national commemorative day, along with the founding year of Dangi (檀君紀元), as the official foundation date of Korea. The date of October 3, which until then had been based on the lunar calendar, was also changed to the solar calendar.

강원도 태백산의 태백제

Taebaekje Ritual in Taebaeksan, Gangwon-do
Cheonjedan Alter in Taebaeksan	Beonsirye Ritual	Incense Burning
Choheonrye Ritual	Jongheonrye Ritual	Soji Haengryeol Procession

Every year on National Foundation day a commemorative ceremony is held at the Dangun Shrine (檀君祠堂) in Sajik Park in Seoul and at shrines all over the country, while the "Cheonje" ceremony is performed on the summits of mountains such as Manisan and Taebaeksan. What's unique about this day is that North Korea recently designated National Foundation Day as a commemorative day and began to organize commemorative events accordingly.