Chinese Calendar Explained: Lunisolar, Not Just Lunar

Diagram showing Earth orbiting the Sun and the Moon orbiting Earth, with lunar phases from new moon to full moon and back.

Chinese New Year, the Lantern Festival, the Dragon Boat Festival and Mid-Autumn Festival are familiar parts of Chinese culture. Behind them lies a calendar that determines when these celebrations take place.

Mid-Autumn falls on the fifteenth day of the eighth “lunar month.” Chinese New Year begins on the first day of the first. In English, the calendar behind these dates is commonly called the lunar calendar.

The name is familiar, but it does not tell the whole story.

The traditional Chinese calendar follows both the Moon and the Sun. Lunar phases establish the months, the Sun helps place those months within the seasonal year, and leap months bring the two cycles into alignment. More precisely, it is a lunisolar calendar.

To understand the difference, start with a simple question: if the calendar followed only the Moon, why would Mid-Autumn remain an autumn festival in China, rather than gradually drifting into spring or summer?

In this article

The Moon Determines When a Month Begins

Months in the Chinese calendar are closely connected to the cycle of lunar phases.

From one new moon to the next takes, on average, about 29.53 days. The astronomical new moon is known in Chinese as shuo (朔). This is not the first thin crescent visible in the evening sky, but a specific moment in the lunar cycle.

In the modern Chinese calendar, the date on which the new moon occurs in Beijing time—UTC+8—is designated the first day of the month. The month begins at midnight at the start of that date, not at the exact moment of the new moon.

Successive month-start dates are either 29 or 30 days apart. This gives the calendar its shorter, 29-day months and longer, 30-day months. They do not alternate in a strict pattern, and a particular numbered month is not permanently assigned one length.

A calendar date is also different from an astronomical moment. The fifteenth day usually falls close to a full moon, but it is not necessarily the date on which the Moon reaches its fullest phase.

The Sun Places the Months Within the Seasons

Lunar phases alone cannot keep months aligned with the seasons.

The Chinese calendar also uses twenty-four solar terms, including the Beginning of Spring, Spring Equinox, Summer Solstice and Winter Solstice.

Although solar terms often appear in Chinese calendar listings alongside lunar dates, they are not determined by the Moon. In the modern system, each term corresponds to a particular position of the Sun along the ecliptic—its apparent annual path against the stars. Successive terms are separated by 15 degrees of solar longitude.

The Sun also helps determine how months are numbered. One important rule is that the lunar month containing the winter solstice is designated the eleventh month.

“When does this month begin?” and “Which numbered month is it?” are therefore different questions. The first is answered by the new moon. The second requires placing the month within the calendar’s solar and seasonal framework.

The eighth month is not simply an arbitrary eighth turn of the lunar cycle. It belongs to a sequence designed to remain connected to the seasonal year.

Why Some Years Have Thirteen Months

The lunar cycle and the seasonal year do not fit neatly together.

Twelve lunar cycles add up to approximately 354 days. A complete seasonal year lasts approximately 365 days, leaving a difference of about eleven days.

If every year contained exactly twelve lunar months, the months would gradually move away from their original seasons. Within a few years, the accumulated difference would approach a whole month.

The Chinese calendar addresses this by adding a leap month in certain years. Some years therefore contain twelve months, while others contain thirteen.

A leap month does not become a separately numbered “month thirteen.” Instead, it repeats the number of the preceding month. The sixth month, for example, may be followed by a leap sixth month, and then the seventh.

Its placement is not arbitrary.

Of the twenty-four solar terms, twelve are known as principal solar terms, or zhongqi (中气). These include the equinoxes and solstices. If there are thirteen lunar months from one winter-solstice month up to, but not including, the next, the first month in that sequence without a principal solar term becomes the leap month.

There is no need to memorize the calculation to understand its purpose: leap months reconcile lunar months with the seasonal year.

This is what makes the calendar lunisolar, rather than purely lunar.

How Festivals Fit the Calendar

Once the roles of the Moon and Sun become clear, festival dates are easier to understand.

Chinese New Year, also known as the Spring Festival, falls on the first day of the first month. The first day is linked to the new moon, but identifying which month is the first requires the calendar’s solar framework and leap-month rules. Chinese New Year is therefore neither determined by the Moon alone nor fixed to the Beginning of Spring solar term.

Mid-Autumn Festival falls on the fifteenth day of the eighth month. Although moon-gazing is central to the celebration, its date is not chosen each year by finding the exact full moon. The festival keeps its calendar date, while the astronomical full moon may fall on a nearby day.

Qingming works differently. It follows the Qingming solar term, rather than a fixed lunar month and day. Qingming is both a seasonal marker and a festival associated with grave-sweeping, remembrance and spring outings.

Traditional festivals do not all follow the same kind of date.

Chinese New Year and Mid-Autumn keep fixed dates in the Chinese calendar, so their Gregorian dates change from year to year. Qingming follows a solar term, making its position in the Gregorian calendar relatively stable.

Where Does This Calendar Come From?

The Chinese calendar was not invented in its entirety by one person in one year. It developed through centuries of observation, calculation and reform.

As early as the Shang dynasty, oracle-bone inscriptions recorded months, differing month lengths and additional months. The practice of counting lunar months and inserting leap months therefore predates the later calendars known by specific names.

In 104 BCE, the Taichu calendar was introduced during the reign of Emperor Wu of the Han dynasty. This important reform established the first month as the beginning of the year, among other changes. It was a milestone in the calendar’s development, not its original starting point.

In 1645, the Qing dynasty officially introduced the Shixian calendar. This reform incorporated Western astronomical knowledge and adopted a method of defining solar terms according to the Sun’s actual position along the ecliptic. Some of the modern calendar’s core organizing principles continue the framework established in this period.

That does not mean today’s calendar still uses the calculations of 1645 unchanged.

Modern calculations use improved astronomical models, updated data and modern time standards. China’s national standard governing the calculation and publication of the traditional calendar was issued in 2017 and took effect on September 1 that year. It formalized modern calculation requirements; it did not mark the invention of the calendar.

Today’s calendar combines a historical framework with the tools of modern astronomy.

Korea, Vietnam and Japan: A Shared Heritage, Different Paths

The influence of this calendar tradition extends beyond China.

Korea has a long history of using calendars derived from the Chinese tradition. During the Joseon dynasty, the Shixian calendar was adopted in 1653. Today, Seollal, the Korean New Year, falls on the first day of the first month of Korea’s lunisolar calendar. Chuseok falls on the fifteenth day of the eighth month.

Vietnam’s traditional calendar also combines lunar phases, solar terms and leap months. Tết celebrates the beginning of the new year according to the Vietnamese calendar, with its own traditions of family reunion, ancestor remembrance and festive food.

A shared heritage, however, does not mean that every place uses one identical calendar.

Local calendar calculations and time standards matter. The astronomical new moon occurs at a single moment, but that moment may fall on different dates in different time zones. When it occurs near midnight, this can shift the first day of a month—and sometimes New Year itself—by a day. The Korea Astronomy and Space Science Institute has explained how this can produce different New Year dates in China and Korea.

Japan followed a different path. In 1873, it adopted the Gregorian calendar as its official civil calendar. Many traditional celebrations were subsequently assigned Gregorian dates. New Year is now primarily celebrated on January 1, while Tanabata usually takes place on July 7, with some communities holding celebrations in August.

This is not the same as converting an old-calendar date into its Gregorian equivalent every year. It is a different way of scheduling the celebration.

There are also exceptions. Japan’s Mid-Autumn moon-viewing observances and “traditional Tanabata” still refer to dates in the old lunisolar calendar. It would therefore be misleading to say that all Japanese traditional festivals have become detached from that calendar.

More Than a Lunar Calendar

Calling the Chinese calendar a “lunar calendar” is common shorthand. In everyday conversation, it usually makes the intended meaning clear.

But to understand how the calendar works, the name is incomplete.

The Moon provides the rhythm of the months. The Sun provides their seasonal setting. Leap months coordinate cycles that do not divide evenly into one another. Festivals connect those dates with family life, customs and cultural memory.

Understanding this calendar is about more than converting dates. Different cultures use the Gregorian calendar to organize everyday life, while many traditional festivals continue to follow their own calendar traditions. People can share a way of counting days and still preserve distinct occasions for celebration, reunion and remembrance—carrying their customs and cultural memory forward.

References

The following sources informed this article. Non-English materials are marked by language, with descriptive English labels provided for readability.

Astronomy and Calendar Rules

History and Festivals

Regional Calendar Traditions

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