The traditional Chinese calendar is lunisolar: its months follow the moon, beginning at the new moon, while an intercalary month is added about every three years to keep the year in step with the sun and hold the solar terms in their seasons. An ordinary year of twelve months runs 353 to 355 days and a leap year of thirteen runs 383 to 385. Its most distinctive feature is the reckoning of years, months, days, and hours by the sexagenary cycle, the sixty combinations formed by pairing the ten Heavenly Stems with the twelve Earthly Branches, which also carry the familiar twelve-year animal zodiac. The calendar governs the Lunar New Year, the traditional festivals, and the almanac used to choose auspicious days. These are the features of the Chinese calendrical tradition, which also underlies the calendars of Korea, Vietnam, and elsewhere in East Asia.
The rule for inserting the intercalary month is fixed by astronomy rather than by a simple periodic count: a leap month is added whenever a lunar month contains no zhongqi, a major solar term, of the twenty-four that divide the solar year; because the solar terms average slightly longer than a lunar month, such a term-less month occurs roughly once every two to three years and is designated a repeat of the month before it, for example a second Fourth Month. This rule was formalised under the Han dynasty's Taichu calendar of 104 BCE and refined through successive imperial reforms, most recently the Shixian calendar introduced under Jesuit astronomical influence in 1645, which remains the algorithmic basis of the calendar's modern civil computation.