How the Chinese Calendar Works
Calendar structure
The Chinese calendar is lunisolar. Its months follow the Moon, while occasional leap months keep the year aligned with the solar seasons. The Gregorian calendar follows the solar year, but its months do not track lunar phases.
Twelve lunar months total about 354 days, roughly eleven days less than a solar year. Without adjustment, the same lunar date would move progressively earlier through the seasons. Adding a leap month produces a thirteen-month year of about 384 days. The combination of shorter and longer years keeps lunar dates within the same broad part of the seasonal cycle.
Months are numbered from one to twelve. A leap month repeats the preceding month's number and carries the label leap, so a thirteen-month year still ends with month twelve. The extra month has its own days, beginning again at day one.
Chinese New Year, also called Spring Festival, begins on day one of lunar month one. In modern use it falls in January or February, on a different Gregorian date from year to year. A lunar year therefore overlaps two Gregorian years. Its beginning is separate from Beginning of Spring, a solar term that marks a point in the seasonal cycle.
Lunar months
Each lunar month begins on the day containing the astronomical new moon and lasts 29 or 30 calendar days. The cycle of lunar phases averages about 29.5 days. A new moon is the conjunction of the Sun and Moon, calculated from their positions in the sky. The calendar does not wait for a visible crescent.
The first day begins at midnight at the start of the new moon's reference date, even when the conjunction occurs later that day. The next new moon's date starts the following month. Month lengths therefore depend on the dates of successive conjunctions, rather than a fixed alternation of 29 and 30 days.
Lunar days are numbered from one through twenty-nine or thirty. A lunar date gives a rough indication of the Moon's phase: it waxes during the first part of the month and wanes during the latter part. Full moon occurs near the middle, but not necessarily on the fifteenth. The astronomical cycle varies in length, and its phases need not fall at the same hour or on the same numbered day each month.
Solar terms and month numbering
The solar part of the calendar consists of 24 solar terms. Each marks another 15 degrees along the Sun's apparent annual path. They follow the seasons independently of lunar-month boundaries, so a solar term can fall early, late or midway through a lunar month. Their Gregorian dates change relatively little from year to year.
Alternate solar terms form the twelve principal terms, which are used to place leap months. In the modern calendar, the lunar month containing Winter Solstice is always month eleven. Counting the months between successive solstice-containing months establishes whether an extra month is needed. When there are thirteen, the first month in that interval without a principal term becomes the leap month. The remaining months receive their numbers in sequence.
This connection to the Sun explains how a calendar with lunar months can keep its year aligned with the seasons. The new moons establish month boundaries, while the solstice and principal terms govern month numbering and intercalation.
Dates and almanac entries
On a modern daily calendar page, first identify the Gregorian date used for ordinary scheduling, then read the corresponding lunar year, month and day. The two dates identify the same day in different calendars. Keep any leap-month marker with the month: removing it changes the date's meaning. A complete recorded lunar date needs the year as well as the month and day.
A page may also display Ganzhi, the stem-and-branch cycle. Its sixty combinations provide repeating names for years and days, alongside the numbered dates.
Solar terms mark the seasonal cycle. Yi lists suitable activities and Ji lists activities to avoid according to almanac tradition. These activity lists do not determine the lunar date.