Why Lunar Months Have 29 or 30 Days: Dates, Seasonal Meaning and Local Customs

Published by Anthony
on November 14, 2026

See why Chinese lunar months have 29 or 30 days, how new moons set the dates, and how to count a short month and a long month in the 2027 calendar table.

Quick Answer: Why Are Lunar Months 29 or 30 Days?

A Chinese lunar month (农历月, nónglì yuè) starts on the civil date of the astronomical new moon (朔, shuò). Successive new moons are about 29.53 days apart on average. Counting whole calendar days from one month’s opening date to the next therefore gives either 29 or 30 days. ^[1][2][3]

Short month: A short month (小月, xiǎoyuè) ends on day 29. ^[1]
Long month: A long month (大月, dàyuè) includes day 30. ^[1]
Calendar rule: The current Chinese calendar uses Beijing time to assign the new moon a civil date. ^[2][3]

Find the next day one and count up to the day before it. ^[2][4]

Four illustrated notes on lunar months of 29 or 30 days

In the Hong Kong Observatory’s 2027 table, August 1 is lunar day 29 and August 2 begins month seven. There is no day 30 in month six. August 31, on the other hand, is lunar day 30 in month seven. Both rows follow the same rule for the next month’s opening date. ^[4]

A New Moon Sets the Date of Day One

Astronomers determine the new moon from the apparent positions of the Sun and Moon. Under the Chinese calendar standard, the instant is assigned to a date in Beijing time. If a new moon occurs in the evening, day one still starts at midnight at the beginning of that date. Almanacs can publish the opening date in advance by calculating that astronomical instant. ^[1][2][3][5]

The time from one new moon to the next averages about 29.53 days, and actual intervals vary. Calendar months use whole civil days. Depending on the dates on which the new moons occur, their month starts are 29 or 30 calendar days apart. The numbered months can therefore have different lengths in different years. ^[1][2][5]

How to Count a 29-Day and a 30-Day Month

The Hong Kong Observatory’s 2027 conversion table shows how to count two consecutive months. The date under the next month heading belongs to the next month, so the count ends on the previous civil day. ^[4]

Lunar month	Day one	Last day	Next month begins	Length
Month six	July 4	August 1	August 2	29 days
Month seven	August 2	August 31	September 1	30 days

Subtract the opening date from the next opening date. There are 29 days from July 4 to August 2 and 30 days from August 2 to September 1. The Gregorian and lunar date rows label the same civil days in two ways. ^[4]

What Short Months Mean for Festival Dates

A festival on day 15 has a date in either kind of month. An observance on the last day has to follow that month’s actual length. Lunar New Year’s Eve (除夕, Chúxī) falls on the final day of the year, whether that day is lunar 29 or 30. The eve is February 16 in 2026, the 29th day of the last month, and February 5 in 2027, also day 29. In the 2024 calendar it was February 9, day 30. ^{[4][6][7][8][9]}

A leap month adds a whole month when the rules call for it. Its day count still comes from the dates of the successive new moons. A leap month can therefore be either 29 or 30 days long. ^[1][2][3]

FAQs About Chinese Lunar Month Length

Do short and long lunar months always alternate?

The lengths follow the dates of the new moons. Consecutive months can have the same length. In 2027, lunar months nine and ten each have 30 days. ^[1][4]