After 2024, we will experience 8 Lunar New Years without a 30th day of the last month. The New Year’s Eve moment will occur on the night of the 29th day of the last month. Why is this strange phenomenon happening?
With just over 7 weeks until the Lunar New Year of the Year of the Serpent 2025, many people have started checking the new year’s calendar and discovered an interesting phenomenon through the perpetual calendar. Not only will the upcoming traditional Tet celebrations lack a 30th day of the last month, but for the next 7 years, the lunar December will only have 29 days. In other words, for 8 consecutive years starting from the Year of the Serpent, we will not experience a 30th day of the last month.
In 2025, there is no 30th day of the last month. (Photo: Perpetual Calendar).
Why does the last month sometimes have 29 days and sometimes 30 days?
Master Tran Tien Binh, a former official of the State Calendar Department, later worked at the Research Department of Calendars, Information Documentation Center, Vietnam Academy of Science and Technology, and the author of “The Vietnamese Calendar in the 20th-21st Century“, analyzes: The first day of the lunar month is the day when the Earth, Moon, and Sun are aligned; with the dark side of the Moon facing the Earth.
In ancient folklore, this is known as the new moon day or the day of the solar eclipse. The solar eclipse point (Earth, Moon, Sun aligned) falls on any date (at any time of the day, from midnight to 24 hours), then that day is the first day of the lunar month.
The lunar month is the interval between two solar eclipse days. The length of the lunar month only varies between approximately 29.27 days to 29.84 days, averaging 29.53 days, and calendar makers round this to 29 days (short month) and 30 days (full month). This leads to the phenomenon where some years do not have a 30th day of the last month.
It is quite common to have consecutive years with or without a 30th day of Tet. For example, from 2014 to 2021, the Lunar New Year consistently had a 30th day. Then came 2022 (Year of the Tiger) with a short last month, followed by 2023 (Year of the Cat) and 2024 (Year of the Dragon) which both had a full last month. This will be followed by 8 consecutive years ending the lunar calendar year with the 29th day of the last month.
Master Tran Tien Binh provides an example of how the solar eclipse point affects the Lunar New Year: In 1968, Vietnam celebrated Tet a day earlier than China because Vietnam uses the GMT+7 time zone, while China uses GMT+8.
December 31, 1967, corresponds to the first day of the last month in both the Vietnamese and Chinese calendars, but the solar eclipse point occurred at 11:29 PM on January 29, 1968, according to Vietnamese time. Therefore, the last month in the Vietnamese calendar had only 29 days (short month). In Beijing time, this moment was 12:29 AM on January 30, also the 30th day of the last month. Thus, the first day of Tet in 1968 in Vietnam arrived one day earlier than in China.
In 1968, Vietnam welcomed the New Year after the 29th of the last month, while China still had a 30th day of Tet.
Why will there be no 30th day of Tet from 2025 to 2032?
According to the perpetual calendar, the Year of the Dragon will end on the 29th of the last month, corresponding to January 28, 2025. The moment of the Lunar New Year will occur right after the last moment of the 29th day of the last month passes. The first day of Tet will be January 29 in the Gregorian calendar.
The Year of the Serpent marks the beginning of a series of 8 consecutive lunar years, where the last month only has 29 days, lasting until 2032. It isn’t until the Lunar New Year of the Year of the Ox in 2033 that we will encounter a 30th day of Tet again. The absence of a 30th day of Tet for many years raises questions: is there a rule governing this phenomenon?
As mentioned earlier, whether a particular year’s last month has 29 or 30 days is not determined by the subjective arrangement of calendar makers but is based on identifying the solar eclipse point. According to Master Tran Tien Binh, the occurrence of years without a 30th day of the last month does not follow a specific pattern, as the calculation of the calendar is based on the complex movements of celestial bodies and the Moon. The Moon is influenced by the gravitational pull of the Sun, Earth, and many other planets, causing the solar eclipse point—related to short and full months—to also fluctuate without following any definite cycle.
For 8 consecutive years starting from 2025, the Lunar New Year will not have a 30th day.
Therefore, in response to the question of why there will be no 30th day of Tet for 8 consecutive years starting in 2025, we can only say that in those years, the solar eclipse point for the last month falls on the 29th. As for why the solar eclipse point of the last month is repeatedly on the 29th for so many years, even calendar makers cannot explain it.
Even a few seconds of miscalculation by astronomers can turn a short month into a full month and vice versa. Even the Zijiang Astronomical Observatory (China) made mistakes in their published calendar in 2002. On September 28, 2057, the solar eclipse point falls at 11:59 PM; on September 4, 2089, it falls at 11:57 PM; and on August 7, 2097, it falls at 11:59 PM, but due to astronomers miscalculating by a few seconds, the solar eclipse point shifted to the next day, altering the length of the month.
Mr. Binh noted that because lunar calendars are so difficult to predict, it is not feasible to statistically analyze the rules of months with 30 days in the last month. The calendars of the years depend on precise calculations at each specific moment. For instance, from 2033 to 2037, there will be full last months, while from 2038 to 2040, there will be short last months, and then there will be a full last month again in 2041.
