Chinese calendar dates behave differently from Gregorian ones in three visible ways: months don’t have a fixed length, Chinese New Year lands on a different Gregorian date every year, and every two or three years a whole extra month appears. None of this is arbitrary. It follows a precise rule for keeping a lunisolar calendar — one that tracks the Moon and the Sun at the same time — from drifting away from the seasons.
Lunisolar, not lunar#
“Lunar calendar” is the common English name, but it undersells what the calendar actually does. A purely lunar calendar (like the Islamic Hijri calendar) tracks only the Moon and lets its months drift freely against the seasons over a 33-year cycle. The Chinese calendar tracks both: its months follow the Moon, but a whole extra month is periodically inserted to keep the year anchored to the Sun. That insertion is the mechanism this article is about.
How a month is set: the new moon#
Each Chinese calendar month begins on the day of the new moon — the moment the Moon sits between the Earth and the Sun and is invisible from Earth — and runs until the next new moon, 29 or 30 days later. Twelve such months add up to about 354 days, roughly 11 days short of a solar year (365.24 days). Left uncorrected, that 11-day gap would compound: within about 16 years, the “first month” would have drifted a full season, from spring into winter. Correcting for that drift is the entire reason a leap month exists.
How a year gets an extra month: the no-major-term rule#
The correction hooks into the same solar framework as the 24 solar terms. Those 24 terms alternate between two kinds: 12 minor terms (jie, 节) and 12 major terms (qi, 气, also called zhongqi 中气) — one major term about every 30.4 days on average. Because an average lunar month (29.5 days) is shorter than the average gap between major terms (30.4 days), the two rhythms slowly fall out of step. Occasionally a lunar month is short enough, and lands awkwardly enough, that it contains no major term at all — squeezed entirely between two of them.
That gap is not treated as a normal skip. The first lunar month in a year to contain no major term is designated the leap month. It is not given a new number; it repeats the number of the month before it, marked with a 闰 (run, “intercalary”) prefix — leap Sixth Month, leap Fifth Month, and so on. Inserting it brings that year to 13 lunar months and roughly 384 days, which pulls the calendar’s drift back to within about a day of the solar year. A leap month occurs in 7 years out of every 19 — a matching cycle between solar years and lunar months that was already known to Chinese astronomers by the Warring States period, and independently to Babylonian and Greek astronomers as the Metonic cycle.
Why Chinese New Year moves#
Chinese New Year is fixed to a lunar point — the new moon that starts the first lunar month — not a Gregorian date. Because that new moon can fall anywhere within a roughly 29-day window each year, and because a leap month occasionally pushes the following year’s new moons later still, the holiday ranges between late January and mid-to-late February on the Gregorian calendar. It is not “random”; it is the direct, visible consequence of the same new-moon and leap-month rules described above.
Worked example: the 2025 leap Sixth Month#
The most recent leap month fell in 2025 (yisi 乙巳 year), inserted after the regular Sixth Month, because that particular lunar month contained no major solar term. The result was two consecutive Sixth Months on the calendar — an ordinary Sixth Month followed by a leap Sixth Month — before the calendar moved on to the Seventh Month. This site’s own almanac tool marks that repeat explicitly: stepping through the days of mid-2025 shows the Sixth Month, then a second month also labeled Sixth Month but flagged as leap, rather than jumping straight to the Seventh. The next leap month, a leap Fifth Month, is due in 2028.
A common misconception#
Calling it a “lunar calendar” suggests the year length floats freely with the Moon, the way the Islamic calendar’s does. It doesn’t. The leap-month mechanism exists specifically to prevent that drift, which is why Chinese New Year always falls in a narrow window near the start of the solar year rather than cycling slowly through all four seasons the way a purely lunar new year does.
Classical source#
The idea that an inserted month is what keeps a lunar calendar aligned with the four seasons is already stated in the Book of Documents (Shangshu, 尚书), in its “Canon of Yao” (Yao Dian, 尧典) chapter: “暮三百有六旬有六日,以闰月定四时成岁” — “the year runs three hundred sixty-six days; the four seasons are fixed, and the year completed, by means of the intercalary month.” The astronomical technique for calculating exactly which month to insert has been refined many times since — the no-major-term rule described above is the version used from the Han-dynasty Taichu calendar reform onward — but the basic insight, that a leap month is what ties the lunar year to the solar seasons, is already on record there.
Related: Chinese Almanac Calendar tool · The 24 Solar Terms · back to Foundations