설날 (aka Seollal) is on the 1st day of the 1st month of the lunar calendar and is may be more recognizable as Lunar New Year in Korea. Like Chuseok, the pre and post days are also included in the holiday and all are family intensive days that drives every Korean to return home in a country wide mass migration. The celebrated ghost towns phenomenon is in full effect and so it's a great time to wander the streets and see some sites, provided they're open.
But it's also the day where all Koreans age a year:
Several East Asian cultures, such as Chinese, Japanese, Korean, and Vietnamese, share a traditional way of counting a person's age. Newborns start at one year old, and each passing of a New Year, rather than the birthday, adds one year to the person's age; this results in people being between 1–2 years older in Asian reckoning than in the Western version. Today this system is commonly used in Koreans' daily life, with exceptions to the legal system and newspaper.So happy birthday, Korean people.