What the lunar calendar is: lunisolar
The Chinese calendar honours two astronomical facts at once: the phases of the moon (synodic months) and the sun (the tropical year). Months are defined by the moon - the 1st is always the dark new-moon day and the 15th is near the full moon; the year is defined by the sun - the month containing the winter solstice is fixed as month 11, and solar terms mark month boundaries. It is therefore a lunisolar calendar: a moon-based month plus a sun-based year. A purely lunar calendar drifts about 11 days per year against the seasons; the lunar calendar locks that drift out with leap months, keeping Chinese New Year in late winter or early spring.
Big and small months: the moon decides
One synodic month averages about 29.53 days, so a lunar month has either 30 days (big) or 29 days (small) - never 31. Which months are big and which are small depends on the true interval between successive new moons: because the lunar orbit is elliptical, the synodic month wavers between about 29.27 and 29.83 days, so there is no fixed pattern and the sequence must be computed year by year. This is why a lunar date cannot be derived from the previous month by mental arithmetic - you need a calendar table.
Where leap months come from: seven in nineteen
Twelve synodic months total about 354 days, roughly 11 days short of a tropical year (about 365.24 days). Left alone, New Year would drift into summer within a few years. The fix is to add a whole extra month every two or three years - the leap month - seven of them in nineteen years on average. Which month is doubled is not arbitrary: a month that contains no "major solar term" (the even-numbered terms of the twenty-four) becomes the leap month, e.g. a leap 4th month follows the ordinary 4th month whenever that extra month has no major term. Lunar years thus alternate between about 354 and 384 days, and the long-run average matches the tropical year.
Why New Year Eve sometimes vanishes
If the 12th lunar month (Layue) is a small month of 29 days, there is no "30th of Layue" and New Year Eve falls on the 29th. This is not an anomaly but a direct consequence of alternating month lengths - it simply happens whenever Layue is small. In recent years some New Year Eves have had no 30th, which is often mistaken for a calendar error. It is perfectly normal: the essence of New Year Eve is the last night of the lunar year, whether that is the 29th or the 30th.
Why Chinese New Year moves on the Gregorian calendar
Chinese New Year is the 1st of month 1, which must fall near the second new moon after the winter solstice; it therefore wanders between 21 January and 20 February on the Gregorian calendar, a span of nearly a month. When a leap month occurs, that lunar year lasts about 384 days and the next New Year comes noticeably later. For the same reason, lunar festivals such as the Dragon Boat Festival and Mid-Autumn shift every Gregorian year, while Qingming, defined as a solar term, stays within 4-6 April.
When does the zodiac change
Two conventions exist for switching the zodiac year: the lunar New Year and Lichun (the "Start of Spring" solar term). For births between the two dates the conventions disagree: someone born on 3 February 2024 is still a Rabbit by the New Year convention (Spring Festival was 10 February) but already a Dragon by the Lichun convention (4 February). Both are genuine, long-standing traditional rules that differ as cultural usage; our zodiac tool lists both side by side.
想马上用：打开 农历万年历 查任意日期的农历与干支，用 生肖查询 对照两种生肖口径，或看 农历节日速查表（2024–2036 逐年推算）。Try it now: look up any date in the lunar calendar tool, compare both zodiac conventions in the zodiac finder, or browse the lunar festival reference (2024–2036).
FAQ
Is the lunar calendar the same as a moon calendar?
Colloquially they are conflated, but strictly no. A moon calendar tracks only the moon and drifts against the seasons. In the Chinese calendar months follow the moon while the year and solar terms follow the sun, reconciled by leap months - a lunisolar system. The twenty-four solar terms are its solar component and have nothing to do with moon phases.
How do people born in a leap month celebrate birthdays?
A leap month recurs only about once in nineteen years, so in most years there is no matching lunar month. Common practice is to celebrate on the corresponding ordinary month (a leap 4th month birth maps to the 4th month), or to use the Gregorian birthday instead. Both are personal choices with no calendrical right or wrong.
Why can lunar months not be worked out with a rule of thumb?
Because big and small months depend on the true moment of each new moon, and that moment is perturbed by the uneven lunar orbit with no fixed pattern; leap months additionally depend on the distribution of major solar terms. Tables must therefore be computed astronomically year by year - which is exactly how our lunar calendar tool works.