What type of calendar determines the traditional date of Chuseok?

A lunisolar calendar determines the traditional date of Chuseok.

The Korean traditional calendar combines lunar months with adjustments that keep the months aligned with the solar year. Chuseok falls on the fifteenth day of the eighth month in that system, so its date moves from year to year on the Gregorian calendar.

This is why Chuseok generally occurs in September or October rather than on one fixed Gregorian date. The same calendar principle affects other traditional Korean holidays, including Seollal. A lunar month is based on the Moon’s cycle, while the added calendar adjustments prevent the holidays from drifting through the seasons.

Calling the system simply a “lunar calendar” is common in everyday English, but “lunisolar” is more precise. A purely lunar calendar would move steadily through the solar seasons; the Korean traditional calendar uses intercalation to preserve seasonal alignment.