Why is there no Tet holiday for 8 consecutive years?

Why is there a 29-day lunar month?

According to the lunar calendar, the number of days in a month is calculated based on the astronomical numbers of the Earth, Moon and Sun. Many people also call the lunar calendar the lunar calendar because it follows the observation of the full moon cycle.

The moment when the Earth - Moon - Sun are in a straight line, the observer standing on Earth can no longer see the Moon, that is the 1st day (New Moon day).

When the Moon - Earth - Sun are in a straight line, it is a full moon.

Although the full moon day (15th day of the lunar calendar) is not necessarily the full moon day, the first day is always the New Moon day.

The way to make the lunar calendar is quite detailed and complicated, determining the cycle from full moon to waning moon is 29.53 days. Therefore, after rounding, there will be 30-day months and 29-day months (also known as short months).

2025 has 2 lunar months

Normally, a solar year has 12 months with 365 days; a lunar year also has 12 months but only 354 days. Compared to the solar year, the lunar year is 11 days shorter, so every 3 years there will be a difference of 33 days, or more than 1 month.

To balance the time between the lunar year and the solar year, an extra leap month must be added every 3 lunar years. However, the lunar year will still be slower than the solar year. People overcome this situation by adding a leap month every 19 years, every 2 years.

In 19 solar years, there are 228 solar months, corresponding to 235 lunar months, 7 months more than the solar year, called 7 leap months. Those 7 leap months are conventionally assigned to the 3rd, 6th, 9th, 11th, 14th, 17th, 19th years of the 19-year cycle.

Therefore, to calculate whether a lunar year has a leap month or not, we take the corresponding solar year and divide it by 19. If it is divisible or gives a remainder of 3, 6, 9, 11, 14, 17, then that year will have a leap month.

The lunar intercalary month is placed in the months without the Middle Qi - a concept derived from the laws of motion of the Earth and the Sun. Of these, January and December are never taken as lunar intercalary months.

With the above calculation, 2025 is a lunar leap year because 2025 divided by 19 has a remainder of 11. Specifically, 2025 is a leap year in the 6th lunar month, meaning there are 2 6th lunar months.

8 consecutive years without Tet 30th

According to the lunar calendar, December this year has only 29 days, no 30th day of Tet. Notably, it will not be until 2033 that there will be a 30th day of December again. That means for 8 consecutive years, from 2025 to 2032, we will not have a real 30th day of Tet.

You can refer to the lunar calendar for Tet holiday in the coming years: