What’s the Deal with Chinese Leap Month

leap monthEver wondered why certain Chinese traditional holidays such as Spring Festival, Dragon Boat Festival, and the Mid-Autumn Day, fall on a different date every year? While you may think this is because they are decided by the Chinese calendar, which is often behind the universal calendar, let us tell you why and how exactly.

The Chinese calendar is sometimes called the lunar calendar, though it is more accurately called a “lunisolar” calendar. It uses the phases of the moon to determine its months and days of the month. In fact, it is quite simple: the first day of the month is always the new moon, the 15th day of the month is always the full moon, and so on. However, the system also uses some mechanisms to keep the months connected to the relevant seasons.

Thus, since there are about 29 days in a lunar month, this would mean a 12-month year would have 354 days. As a result, to keep months and holidays in line with the seasons, the Chinese calendar requires a “leap month” to be inserted approximately every two or three years. These intercalary months are called rùnyuè (leap month) while the year that comes with a rùnyuè is called rùnnián (leap year). But how do they decide when exactly the leap month should be inserted?

Although the rùnnián has 13 months, the extra month is never reffered to as the 13th month. Instead, it is added and named after the first month that’s missing a primary solar term. A solar month is marked by two symbols, the sectional solar term is at the beginning of the month and the primary solar term is in the middle of the month. Due to the lack of about 11 days in every lunar year, the lunar calendar will lose its allignment with the solar terms and leave one lunar month without a primary solar term, every two to three years. When it happens, a leap month will be added after that month and the rùn will be added to the new month to differentiate. For example, a rùn wǔ yuè (rùn of the fifth month) was added in 2009, a rùn sì yuè (rùn of the fourth month) was added in 2012, and in 2014, the rùn jiǔ yuè (rùn of the 9th month) will be added.