How does the Chinese lunar calendar work?

The Chinese lunar calendar is a lunisolar system that tracks both the moon's phases and the sun's position. Unlike the Western Gregorian calendar, it adds a leap month roughly every three years to keep the seasons aligned, which is why Chinese festivals shift dates each year on the Western calendar.

A traditional Chinese almanac (tongshu) showing lunar dates and auspicious directions

Each Chinese lunar month begins on the day of the new moon. A full lunar cycle — from one new moon to the next — takes about 29.5 days, so months alternate between 29 and 30 days. A standard lunar year has 12 months, giving you roughly 354 days. That is about 11 days shorter than the solar year of 365.25 days.

If you only followed the moon, your calendar would drift backward through the seasons over time. The Mid-Autumn Festival, meant for harvest time, would eventually land in spring. To prevent this, the Chinese calendar uses a clever fix.

Every two or three years, the calendar inserts an extra month — called a leap month or runyue (闰月). This makes that year 13 months long instead of 12, bringing the calendar back in sync with the solar year. The decision about which month to repeat is based on astronomical calculations: specifically, which lunar month does not contain a zhongqi (a major solar term). There are 24 solar terms in a year, and the rules for inserting leap months have been refined over centuries by Chinese astronomers.

This is why Chinese New Year can fall anywhere between January 21 and February 20 on the Western calendar. It always lands on the second new moon after the winter solstice.

The Chinese calendar uses two parallel systems to mark years: the 10 Heavenly Stems (tiangan) and the 12 Earthly Branches (dizhi). Combined, they create a 60-year cycle (sexagenary cycle). Each year gets both a stem (one of five elements: wood, fire, earth, metal, water) and a branch (one of 12 zodiac animals).

For example, 2024 was the Year of the Wood Dragon (甲辰). The next Wood Dragon year will not arrive until 2084. This system has been in continuous use for over 2,000 years, making it one of the longest-running calendar systems in human history.

Beyond the lunar months, the Chinese calendar divides the solar year into 24 solar terms (二十四节气). These mark natural phenomena like "Rain Water," "Grain Full," and "Great Cold." Farmers have used these terms for centuries to time planting and harvesting. They are technically solar-based, not lunar, which makes the Chinese system a true lunisolar calendar — one of the most sophisticated ever devised.

Because the calendar is lunisolar, most Chinese festivals shift by 11 to 12 days each year relative to the Gregorian calendar. The Spring Festival (Chinese New Year), Lantern Festival, Dragon Boat Festival, and Mid-Autumn Festival all follow lunar dates. The Qingming Festival is an exception — it is tied to a solar term and always falls around April 4-5.

In daily life, most Chinese people today use the Gregorian calendar for business and scheduling but still consult the lunar calendar for festivals, weddings, and auspicious dates. Many phone calendars in China display both systems side by side.