Gregorian vs Lunisolar Calendar

Most people interact with two calendars simultaneously without realizing it. The Gregorian calendar governs work schedules, legal documents, and international commerce. The lunisolar calendar — shared in varying forms by Korea, China, Japan, and Vietnam — governs traditional festivals, ancestral memorial days (제사), and, for many families, birthdays and auspicious dates.

How Each Calendar Tracks Time

The Gregorian calendar is a purely solar system: it tracks Earth's 365.25-day orbit around the Sun. Leap days every four years (with century exceptions) keep it synchronized. Month lengths are fixed by convention, not astronomical events.

The lunisolar calendar tracks both the Moon and the Sun. Each month begins at the new moon and lasts 29 or 30 days, giving a lunar year of roughly 354 days — about 11 days shorter than a solar year. Without correction, festivals would slowly drift through all seasons. Lunisolar calendars solve this with intercalary (leap) months inserted roughly every 3 years, keeping the calendar aligned with the solar cycle over a 19-year (Metonic) cycle.

Use Lunar Solar to convert any Gregorian date to its lunar equivalent and vice versa, essential for planning ancestral rites, traditional birthdays, and Korean holidays.

Why Holidays Move Each Year

Chuseok (추석) falls on the 15th day of the 8th lunar month — always a full moon, but never on a fixed Gregorian date. It can land anywhere from late September to mid-October on the Gregorian calendar. Similarly, Seollal (설날) and the Chinese New Year shift by up to 4–5 weeks between years.

This "movement" is entirely predictable from the lunisolar calendar — but it requires conversion software or printed tables to calculate, because there is no simple arithmetic rule that maps a lunar date to Gregorian without reference data.

Practical Coexistence

In modern Korea, both systems are actively used. Legal and business dates use Gregorian exclusively. Traditional birthday celebrations (생일) are often held on the lunar date, meaning the Gregorian birthday changes each year. Ancestral memorial services (기제사) follow the lunar death anniversary, requiring families to consult a calendar converter annually.

Japan officially abandoned the lunisolar calendar in 1873 but retains traditional festivals tied to it. China's public holidays include Gregorian New Year and the Lunar New Year (Spring Festival). Vietnam's Tết follows the same lunisolar logic.