No. 136Time4 minviews 16
Published by GK Library Annex
Why does Chuseok fall on a different date every year?
A lunar year is about 11 days shorter than a solar one, and a leap month every few years makes up the gap.
Chuseok, Korea's autumn harvest holiday, lands on a different date each year because it is set by the lunar calendar: the 15th day of the 8th lunar month. Twelve lunar months add up to about 11 days less than a solar year, so on the regular calendar Chuseok creeps a little earlier each year, then jumps well later in a year that gets a leap month.
A month counted by the moon
A lunar month is based on how long the moon takes to return to the same shape, about 29.5 days. While the moon circles the Earth once, the Earth is also moving around the sun, so getting back to the same shape takes about two days longer than one trip around. Since you can't split a day in half, the calendar mixes short months of 29 days with long months of 30.
Put twelve of those months together and you get about 354 days. That's roughly 11 days short of the 365-day solar year. So the same 15th day of the 8th lunar month lands about 11 days earlier on the next year's solar calendar. In fact, Chuseok fell on September 29 in 2023 and on September 17 in 2024.
A leap month pushes it back
If it only ever moved earlier, within a few decades Chuseok would end up in midsummer. The leap month prevents that. The lunar calendar adds an extra month about seven times every 19 years, turning those years into 13 months.
In 2025, a leap 6th month was added after the 6th month. Because of that, Chuseok in 2025 fell on October 6, 19 days later than the year before, and this year, 2026, it moved 11 days earlier again to September 25. By swinging earlier and later like this, Chuseok usually stays between early September and early October. Seollal, the Lunar New Year, moves between late January and mid-February for the same reason.
Not just the moon
Many people think of the lunar calendar as one that only follows the moon, but the calendar Koreans use is a lunisolar calendar that also tracks the sun. Even where the leap month goes is decided by the 24 solar terms, which divide the year by the sun's position. That's why the winter solstice, one of those solar terms, falls around December 21 or 22 on the solar calendar every year, not on a fixed lunar date.
The 15th lunar day isn't always the roundest full moon, either. The moon is often at its fullest a day or two later, on the 16th or 17th. If you celebrate a birthday by the lunar calendar, its solar date changes every year, so it helps to check ahead with the Korea Astronomy and Space Science Institute's lunar–solar date converter.
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Chuseok follows a lunar date, so it moves about 11 days earlier each solar year, then jumps later in a leap-month year.
Notes and references · Korea Astronomy and Space Science Institute calendar data. 29- or 30-day lunar months, the rule of 7 leap months in 19 years, and Chuseok's solar dates for 2023–2026. Leap months are calculated yearly, so their spacing isn't even.
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