A friend of mine got married in Taipei in early autumn 2025 and was confused when her grandmother insisted that the wedding date had to account for whether the year had two sixth months. “What does that even mean?” she asked over text, the day after the family had quietly rescheduled her originally planned outdoor venue to a slightly different week. The answer turned out to involve a 2,500-year-old calendar correction and a piece of celestial mechanics that synchronizes the moon’s phases with the seasons across roughly two human generations. The lunisolar calendar that East Asia has used for millennia doesn’t simply add a thirteenth month whenever it feels behind the sun; it follows a precise 19-year pattern that produces exactly seven leap months in every cycle, and 2025’s Leap Sixth Month was the most recent one. 2026 has no leap month. The next one comes in 2028.
Two calendars, two corrections
Most readers in English-speaking countries encounter calendar corrections only through the Gregorian leap year — the February 29 inserted every four years to keep the calendar aligned with Earth’s actual orbital period. The Gregorian system tracks the sun. Its correction is small (one day per four years, with refinements every century and four centuries) and it accumulates over a multi-century timeline.
The East Asian lunar calendar tracks both the moon and the sun. That’s the source of its complexity. A purely lunar calendar — one that just counts moon phases without trying to stay aligned with seasons — would drift by about 11 days per solar year, so the months would shift across seasons rapidly: Lunar New Year in spring during one decade would migrate through summer, autumn, and winter over a roughly 33-year cycle. The Islamic calendar works this way, which is why Ramadan moves backward through the seasons.
The East Asian calendar instead anchors itself to the seasons by inserting an extra month — a leap month, also called an embolismic or intercalary month — at intervals that average a little more than two years. With those leap months, the calendar holds a stable relationship to the seasons over centuries. Wikipedia’s lunisolar calendar article summarizes the structure: 12 lunar months per ordinary year, 13 per leap year, with the leap months distributed to keep the lunar months aligned with the solar terms.
The Metonic cycle — 19 years, 235 months, 2 hours of error
The mathematical fact that makes the East Asian calendar work is unusually elegant. Nineteen tropical years equal 6,939.6 days. Two hundred thirty-five synodic months (the time between consecutive new moons) equal 6,939.7 days. The two periods differ by only about 2 hours and 5 minutes. This near-coincidence means that the moon’s phases will return to the same Gregorian dates after every 19 years — a fact discovered by the Greek astronomer Meton of Athens around 432 BCE and observed independently by ancient Chinese astronomers.
The implication is that the lunar calendar can be brought back into perfect alignment with the solar calendar every 19 years, with a deviation of only a few hours that accumulates very slowly over millennia. To make 19 lunar years cover 235 lunar months instead of the 228 they would naturally contain, you need to insert 7 extra months over the 19-year span. That’s the source of the 7-in-19 leap-month rule.
Y.T. Liu’s deep technical analysis of the Chinese calendar walks through how the Chinese astronomical bureau implements this rule with precision, calculating each year’s solar terms to the minute and using those calculations to determine where leap months fall.
How the leap month gets placed — the ‘no zhongqi’ rule
Knowing that 7 leap months go somewhere in every 19 years tells you the count. It doesn’t tell you where. The placement rule is called 無中置閏法 in Chinese (Korean: 무중치윤법, Japanese: 無中置閏法) — literally ‘place a leap month where there is no zhongqi.’
Here’s how it works. Each of the 24 solar terms is classified as either a ‘major term’ (zhongqi 中氣, falling on multiples of 30° of the sun’s ecliptic longitude — winter solstice, autumn equinox, etc.) or a ‘minor term’ (jieqi 節氣, falling on the odd 15° increments — beginning of spring, awakening of insects, etc.). There are exactly 12 major terms in a year, one per zodiac sign. In a regular lunar month, exactly one major term falls within the month’s days.
Because the lunar month averages 29.5 days while consecutive major terms are 30.4 days apart, every so often a lunar month will start and end without containing any major term. That month is the leap month, and it gets named after the preceding month with ‘leap’ (閏) prefixed. So ‘Leap Sixth Month’ (閏六月) is a second sixth month inserted between the regular Sixth Month and Seventh Month, in the spot where a major term failed to land.
This rule explains why leap months can show up at any position in the lunar year. Leap January, Leap May, Leap August — all are possible depending on where the no-major-term gap falls. The most common positions are Leap May and Leap June, because the gaps are slightly more likely there given the elliptical shape of Earth’s orbit.
2025, 2026, and 2028 in the cycle
The recent and upcoming pattern:
- 2024: Regular 12-month year. Lunar New Year February 10.
- 2025: 13-month year with Leap Sixth Month. Lunar New Year January 29.
- 2026: Regular 12-month year. Lunar New Year February 17.
- 2027: Regular 12-month year. Lunar New Year February 6.
- 2028: 13-month year with Leap Fifth Month. Lunar New Year January 26.
Notice the see-saw. After a leap year (2025), the next Lunar New Year (2026) falls later in the Gregorian calendar — the leap month pushed the lunar year forward. After a regular year (2026), the next Lunar New Year (2027) falls earlier — the lunar months gain on the Gregorian year by 11 days. The leap pattern keeps Lunar New Year between roughly January 21 and February 21 indefinitely.
Wikipedia’s article on the Chinese calendar lists the historical leap month positions and projects them forward. The pattern doesn’t repeat exactly every 19 years because of small irregularities in Earth’s orbital motion, but it recurs with high reliability across multi-decade windows.
What it means in practice
For most readers in 2026, the leap month rule mostly matters at the cultural and family level. A few examples:
Birthdays in leap months: Someone born in a leap month — say, in 2025’s Leap Sixth Month — has a birthday that doesn’t recur every year. A Leap Sixth Month is a second sixth month, so in a regular twelve-month year the lunar calendar has no Leap Sixth Month slot at all. Some families celebrate the lunar birthday on the regular Sixth Month every year and then on the Leap Sixth Month only when leap years occur. Others double up the celebration on leap years. The decision is family-by-family. To see how this plays out concretely: enter ‘Leap Sixth Month, day N, 2026’ into PiPi Worlds’s lunar calendar tool, and because 2026 is a regular twelve-month year with no Leap Sixth Month, the tool reads the year’s lunar structure and returns the Gregorian date of the regular Sixth Month’s day N instead. Enter a year that does carry a leap month — 2028 with its Leap Fifth Month — and you can see at a glance how the converted date differs between a year that has the leap slot and a year that doesn’t.
Weddings: Some Chinese and Korean families avoid weddings in leap months out of a tradition that ‘leap’ connotes ‘extra’ or ‘unstable.’ Other families take the opposite view — that leap months sit outside ordinary celestial rules and so are auspicious for major life decisions. Both views coexist; the choice is regional and familial.
Funerals and memorial dates: Lunar memorial dates for a deceased ancestor pose a problem if the death occurred in a leap month. The most common solution is to observe the memorial in the equivalent regular month each year (regular Sixth Month) and add an extra observance on years that have the corresponding leap month.
Lunar New Year retail and travel planning: Chinese New Year migrations, the largest annual human movement on Earth, anchor on the lunar New Year date that the leap-month system produces. Travel agencies in China, Korea, and overseas Chinese communities watch the leap pattern carefully because a January 21 Lunar New Year (early end of the range) creates very different travel demand than a February 21 Lunar New Year (late end).
The Japanese exception
Japan moved to the Gregorian calendar in 1873 during the Meiji Restoration and abolished official use of the lunar calendar at the same time. So while Chinese and Korean calendars in 2026 still display leap months in their official versions, Japanese government calendars don’t. The lunar calendar (kyūreki, 旧暦) survives in temple schedules, traditional festivals, and a small print run of specialty calendars that publish lunar dates alongside Gregorian. The calculations match Chinese and Korean lunar dates exactly because they use the same astronomical algorithms.
This means a few cultural curiosities. Japan’s three-day Bon festival, originally a lunar observation, is celebrated on different Gregorian dates in different regions: most of Japan observes ‘July Bon’ (around July 15 of the Gregorian calendar), while some regions including parts of Tokyo observe ‘August Bon’ (around August 15 of the Gregorian calendar) as a closer approximation of the original lunar date. Tanabata, originally lunar, is observed on July 7 nationally but on August 7 in some regions. Japanese tradition essentially preserves the rhythm of the lunar calendar without keeping its calculation as a national time standard.
What Gregorian date is my leap-month birthday this year?
Once the astronomy is clear, one practical question usually remains: if you — or a parent — were born in a leap month, what Gregorian date should this year’s birthday land on? A leap-month birthday doesn’t resolve automatically every year, because in a regular twelve-month year that lunar month simply isn’t on the calendar. Worked through by hand, the steps are: (1) confirm whether the lunar birthday falls in a leap month or a regular month — say, 2025’s Leap Sixth Month; (2) check whether the current year carries a leap month in that position — 2026 is a regular twelve-month year, so there is no Leap Sixth Month slot; (3) convert the same lunar day of the regular Sixth Month into that year’s Gregorian date. All three steps depend on knowing the exact lunar structure of the year in question.
For checking that structure, the leap-month tables published by the Purple Mountain Observatory in Nanjing and the Korea Astronomy and Space Science Institute are the authoritative references, and Japanese kyūreki tables from temples and traditional almanac publishers follow the same calculations.
PiPi Worlds’s lunar calendar tool handles all three steps in one pass. Give it a leap-month birthday — ‘Leap Sixth Month, day N’ — and a year to convert into, and it first determines whether that year is a leap year or a regular year. For a year that carries the leap month (2025, 2028), it returns the Gregorian date of the leap-month slot; for a regular twelve-month year (2026, 2027), it returns the Gregorian date of the regular month instead. The ‘how do you decide it in a regular year’ ambiguity turns into a single-line answer inside the tool. So if you’re trying to figure out what your grandmother meant when she said you were born in a Leap Eighth Month, or which year your wedding has to dodge a Leap Sixth Month, the tool can answer in a single look-up.
Closing — a calendar that bends to the moon and the sun at once
The Metonic cycle’s elegance, for me, lies in how it solves a problem that doesn’t have to be solvable. The moon’s phases and the Earth’s orbit don’t have to align over any reasonable timescale; they could have been off by a non-rational ratio that no whole-number cycle could capture. Instead, they happen to align with a 2-hour error every 19 years, and ancient astronomers — Greek, Chinese, Babylonian — independently noticed this and built calendars around it. The leap month system is the user-facing implementation of that astronomical coincidence. Every two or three years, the calendar inserts an extra month to keep the lunar phases tied to the solar terms, and over a 19-year cycle the count comes out exactly right. 2026 is one of the calendar’s twelve regular years. 2028 brings a Leap Fifth Month. The pattern continues, calibrated to the heavens by a discovery that’s been holding up for two and a half millennia.