The Lunar Calendar and Leap Months, Explained
The Korean lunar calendar is really lunisolar: its months follow the moon, but its year is kept in step with the sun. The tool that keeps them together is the leap month, and haru works it out with two simple rules.
Months that follow the moon
A lunar month begins on the day of the new moon, the moment the moon stands between the Earth and the sun. From one new moon to the next takes a little over 29 and a half days, so months have either 29 or 30 days. In 2026, for example, the month containing 21 June has 29 days, while the month containing Chuseok has 30.
Haru computes each new moon with the series in Jean Meeus's Astronomical Algorithms (chapter 49) and fixes its date in Korean standard time, UTC+9. The date matters: a new moon shortly before midnight in Seoul would already be the next day further east, so the zone is part of the rule.
Twelve moons are not quite a year
Twelve lunar months add up to only 354 or 355 days, roughly eleven days short of a solar year. Left alone, the lunar new year would drift through the seasons. Instead, every two or three years a thirteenth month is inserted and the lunar year grows to 383 or 384 days. You can see this in the gaps between lunar new years below.
- Every year with a leap month is 383 or 384 days long; every year without one is 354 or 355.
- Across these twelve years the lunar new year moves between 23 January (2031) and 19 February (2034).
- Chuseok ranges from 8 September (2033) to 6 October (2025).
| Lunar year | New year (1/1) | Chuseok (8/15) | Days to next new year | Leap month |
|---|---|---|---|---|
| 2024 | 2024-02-10 | 2024-09-17 | 354 | - |
| 2025 | 2025-01-29 | 2025-10-06 | 384 | leap 6 |
| 2026 | 2026-02-17 | 2026-09-25 | 355 | - |
| 2027 | 2027-02-07 | 2027-09-15 | 354 | - |
| 2028 | 2028-01-27 | 2028-10-03 | 383 | leap 5 |
| 2029 | 2029-02-13 | 2029-09-22 | 355 | - |
| 2030 | 2030-02-03 | 2030-09-12 | 354 | - |
| 2031 | 2031-01-23 | 2031-10-01 | 384 | leap 3 |
| 2032 | 2032-02-11 | 2032-09-19 | 355 | - |
| 2033 | 2033-01-31 | 2033-09-08 | 384 | leap 11 |
| 2034 | 2034-02-19 | 2034-09-27 | 354 | - |
| 2035 | 2035-02-08 | 2035-09-16 | - | - |
Rule one: month 11 holds the winter solstice
The anchor is the sun. Twelve of the 24 solar terms are principal terms (zhongqi, 中氣), falling when the sun's longitude is a multiple of 30 degrees. Each numbered month is named after the principal term it contains, and the fixed point is the winter solstice: the month containing it is always month 11. Haru's calculations for 2026 show the pattern holding.
- 2026-06-21, the day of the summer solstice, is lunar 5/7: inside month 5.
- 2026-12-22, the day of the winter solstice, is lunar 11/14: inside month 11.
| Principal term | Sun's longitude | Moment (KST) | Lunar month |
|---|---|---|---|
| 雨水 Rain Water | 330 | 2026-02-19 00:52 | 1 |
| 春分 Spring Equinox | 0 | 2026-03-20 23:46 | 2 |
| 穀雨 Grain Rain | 30 | 2026-04-20 10:39 | 3 |
| 小滿 Grain Buds | 60 | 2026-05-21 09:37 | 4 |
| 夏至 Summer Solstice | 90 | 2026-06-21 17:24 | 5 |
| 大暑 Major Heat | 120 | 2026-07-23 04:13 | 6 |
| 處暑 End of Heat | 150 | 2026-08-23 11:19 | 7 |
| 秋分 Autumn Equinox | 180 | 2026-09-23 09:05 | 8 |
| 霜降 Frost Descent | 210 | 2026-10-23 18:38 | 9 |
| 小雪 Minor Snow | 240 | 2026-11-22 16:23 | 10 |
| 冬至 Winter Solstice | 270 | 2026-12-22 05:50 | 11 |
| 大寒 Major Cold | 300 | 2027-01-20 16:30 | 12 |
Rule two: the leap is the first month without a principal term
Haru counts the new moons between one winter solstice's month 11 and the next. Usually there are twelve months in that span and each one contains a principal term. When there are thirteen, one month must be left without a principal term, and the first such month becomes the leap month. It takes the number of the month before it, so a leap month after month 6 is 'leap 6'.
Between 2024 and 2035 the engine finds four leap months: leap 6 in 2025, leap 5 in 2028, leap 3 in 2031 and leap 11 in 2033. The 2033 case is unusual. The first month without a principal term falls right after the winter solstice month, so the leap month is a second month 11, and month 12 and the next new year come late: lunar new year 2034 falls on 19 February, the latest in the table above.
What this means for birthdays
A lunar birthday lands on a different solar date each year, because the lunar year is shorter or longer than the solar one. Converting a lunar birth date correctly also needs the leap flag: in 2025, month 6 happened twice, and the leap month's day 10 is a different solar date from the ordinary month's day 10. Haru asks whether a lunar date is in a leap month, and if the day does not exist in that month (say, day 30 of a 29-day month) it refuses the date rather than guessing.
One worked example: the lunar date 1970, month 8, day 15 converts to the solar date 1970-09-15, and that solar date is what the pillars are computed from.
Questions
Why do Seollal and Chuseok move every year?
They are fixed in the lunar calendar (1/1 and 8/15), and lunar years are 354, 355, 383 or 384 days long, so their solar dates shift.
What is a principal term?
One of the twelve solar terms at multiples of 30 degrees of the sun's longitude, such as the equinoxes, the solstices and 雨水. Each normal lunar month contains one.
Is a leap month unlucky?
The calendar itself says nothing of the kind. A leap month is simply the adjustment that keeps the months in step with the seasons.
Does the leap month affect the pillars?
Not directly. Month pillars follow the solar terms, not lunar months, so a lunar date is converted to a solar one first.
Read next
Traditional Four Pillars (myeongni) and yin-yang five-element interpretation. Not a medical diagnosis or advice.