TL;DR: A leap month is added to the lunar calendar to make up the difference between the lunar year (about 354 days) and the solar year (about 365 days). A leap month is a month that contains no major solar term; the 19-year Metonic cycle (about 7 leap months in 19 years) is only the average frequency, not the rule for placing them. The 2025 Year of the Snake has a leap sixth month, so it lasts 384 days.
What Is a Lunar Leap Month?
A leap month is a month added to a lunar year to adjust the mismatch between the Moon's cycle and the cycle of the four seasons. In an ordinary lunar year, each month begins on the day of the new moon, called the conjunction point. The time from one conjunction point to the next, that is, one lunar month, averages 29 days 12 hours 44 minutes and can vary by about 7 hours. Therefore, an ordinary lunar year has only 354 or 355 days, about 11 days shorter than a solar year.
If left as is, after just a few years the lunar months would drift relative to the seasons and solar terms. So people long ago added a leap month in certain years to keep the lunar calendar roughly aligned with the cycle of the four seasons. A year with an added leap month is called a lunar leap year, and the total number of days in that year can reach 383 or 384 days.
Comparison table of the characteristics of the lunar calendar, solar calendar, and leap cycle according to the data in the article.
| Characteristic | Lunar calendar | Solar calendar |
|---|---|---|
| Basis | The cycle of the Moon's waxing and waning | The cycle of the Earth's revolution around the Sun |
| Length of a common year | 354 or 355 days | 365 days |
| Length of a leap year | 383 or 384 days | 366 days |
| Rule for adding leap periods | Add 1 leap month where a lunar month contains no principal term; on average 7 leap months in 19 years | Every 4 years, add one day to the end of February |
| Difference each year | Shorter than the solar year by about 11 days | None |
| Equivalent cycle | 235 lunar months are approximately equal to 19 solar years | 19 years are approximately equal to 235 lunar months |
Why Does the Lunar Calendar Need a Leap Month?
The lunar calendar is based on the Moon's cycle of waxing and waning, closely following the change of phases from new moon, crescent, half moon, gibbous, full moon, and back again. Each lunar month begins on the day of the new moon, when the Moon is on the same side as the Sun. This moment is called the conjunction point, and whichever day contains the conjunction point is the first day of the month.
Meanwhile, the solar calendar is based on the Earth's orbit around the Sun. An ordinary solar year has 365 days, but the actual time for the Earth to complete one orbit around the Sun is about 365.24 days, that is, 365 days 5 hours 48 minutes 56 seconds. To make up this difference, the solar calendar applies the leap year rule, adding one day at the end of February every 4 years, bringing the total to 366 days.
The difference between a lunar year and a solar year reaches about 11 days each year. Without adjustment, after only about 3 years the lunar calendar would drift more than a month away from the solar calendar. To synchronize with the cycle of the four seasons and minimize the difference, the lunar calendar system applies the method of adding 1 leap month. This rule helps the lunar calendar stay roughly aligned with the solar calendar while still accurately reflecting the Moon's waxing and waning cycle.
How Is a Leap Month Determined?
A leap month is not chosen arbitrarily but is determined by detailed calculation of the Sun's position on the ecliptic based on the 24 solar terms. In the solar term system, there are 12 major solar terms including Rain Water, Spring Equinox, Grain Rain, Grain Full, Summer Solstice, Major Heat, End of Heat, Autumn Equinox, Frost's Descent, Light Snow, Winter Solstice, and Major Cold, corresponding to the lunar months.
The important principle is: a leap month is assigned to months that contain no major solar term. In other words, if a lunar month (counted from one conjunction point to just before the next conjunction point) contains no major solar term, that month is called a leap month and takes the name of the month immediately before it.
A specific example with the 2025 Year of the Snake: from the conjunction point on June 25 to the conjunction point on July 25 there is the major solar term Major Heat (July 22), so this is an ordinary sixth lunar month. Next, from the conjunction point on July 25 to the conjunction point on August 23, the major solar term End of Heat falls exactly on August 23, and by convention August 23 is the first day of the next lunar month. Thus, the period from July 25 to before August 23 contains no major solar term, and by convention this is a leap month. That is the leap sixth month in 2025.
The Metonic Cycle and the Frequency of Leap Months
The lunar calendar does not place leap months on a fixed cycle but works them out year by year: when 13 lunar months fall between two eleventh months, the first month containing no principal term is the leap month. The 19-year Metonic cycle only describes the average frequency: over the long run, there are about 7 leap months every 19 years. This means that roughly every 2 to 3 years there is a lunar leap year, but not according to a simple fixed rule like the solar calendar (once every 4 years).
That frequency comes from an astronomical near-coincidence: 19 solar years are nearly equal to 235 lunar months. Because leap months follow the principal terms, the lunar calendar stays roughly aligned with the seasons over the long term.
Why Does One Lunar Day in a Leap Year Correspond to Two Solar Days?
This is a common question when consulting a lunar-solar calendar. In a lunar leap year, because an extra leap month is added, some lunar days appear twice in the corresponding solar year. For example, the 2025 Year of the Snake has two sixth lunar months: the ordinary sixth month and the leap sixth month. This leads to a day such as the first day of the sixth month having two different solar dates: one for the ordinary sixth month and one for the leap sixth month.
Specifically, the first sixth lunar month has 30 days, starting from June 25 to July 24 in the solar calendar. Next is the leap sixth month lasting 29 days, from July 25 to August 22 in the solar calendar. Thus, if someone is born on the first day of the sixth lunar month, there are two corresponding solar dates: one is June 25 and one is July 25 (the first day of the leap sixth month). This is the phenomenon of "one lunar day corresponding to two solar days" in a leap year.
This does not mean the lunar day is strangely duplicated, but rather that an extra leap month is inserted, causing the month name to repeat. The days within the leap month still continue seamlessly in terms of time, but in terms of naming they repeat the previous month.
The Influence of the Leap Month on Life and Culture
Adjusting for the leap month not only ensures the lunar calendar accurately reflects the Moon's waxing and waning cycle but also keeps the seasons and solar terms from drifting too far from reality. This is very important for agriculture, festivals, and traditional cultural activities of Vietnamese people.
In agriculture, planting is based on solar terms and seasons. If the lunar calendar drifts too far from the solar calendar, guidance on seasonal timing would become inaccurate. The leap month helps adjust this.
In culture, many traditional festivals are held according to the lunar calendar such as Tết Nguyên Dan, the Mid-Autumn Festival, Tết Hàn Thuc, and so on. Having a leap month does not change the dates of the main festivals, but it can affect secondary occasions or rituals tied to a specific month. For instance, if there is a leap sixth month, activities tied to the sixth month will be held twice in the year.
Common Misunderstandings About the Leap Month
A common misunderstanding is thinking that a lunar leap year is unusual or frightening. In reality, calendar experts affirm that this is normal in the lunar calendar, with nothing strange about it. The 2025 Year of the Snake with 384 days and two sixth months is a typical example, but it is not a rare phenomenon.
Another misunderstanding is thinking that a leap month can fall in any month, including the first month. In reality, a leap month can fall in many different months, but not every month can be a leap month. According to the rule, a leap month is a month with no major solar term, and this depends on the Sun's position on the ecliptic. In the past, there was a year with a leap first month, for example the 1803 Year of the Water Pig according to the Vietnamese calendar. In the future, the 2148 Year of the Earth Monkey is said to have a leap first month, with the first day of the first month falling on Sunday, January 21, 2148, and the first day of the leap first month on Tuesday, February 20, 2148. However, this does not mean Vietnamese people will celebrate two Tết Nguyên Dan, because Tet is counted by the first first month, not the leap month.
