To explain this phenomenon, we need to pay attention to the type of calendar that countries are using. Currently, countries and ethnic groups around the world use 3 main types of calendars: solar calendar, lunar calendar and yin-yang calendar.
In which, the solar calendar takes five-line (i.e., the cycle of the Earth orbiting the Sun) as the standard. The most common is the solar calendar with a year lasting 365 days, a profit year of 366 days, an average of 365,2425 days per year (due to the profit rule: 97 years of profit in a 400-year cycle).
The lunar calendar is based on the ecliptic month, which is the cycle of changes in the lunar phases. This type of calendar does not consider the Earth's movement around the Sun, with only 354 or 355 days per year.
The lunar calendar (agricultural calendar) is a form of the solar and lunar calendar. The number of days in each month is determined according to the lunar phase cycle. The day without a moon is called "soc", the day with a full moon is called "vong".
On average, a month of soc vong is 29.5306 days long, so each lunar month is only 29 or 30 days. Among them, the 30-day month is called "full month", and the 29-day month is "short month".
Notably, because the number of days in a month must be an integer, the lunar month is sometimes a short month (29 days), sometimes a full month (30 days). The arrangement of full months and short months does not follow the fixed law of one full - one short, but may appear many full months or many consecutive short months. This must be based on accurate astronomical calculations.
According to the lunar calendar compilation rules, the 1st day of each month is determined by the time the "soc" appears. Experts give an example if the soc occurs at 11:59 pm, that lunar month will be shortened by one day. But if the soc falls at 0:01 am, that month will be longer by another day. Just a 1-2 minute difference is enough to decide whether New Year's Eve is the 29th or the 30th of Tet.
In 2025, the squirrel falls at 8:36 pm on January 29, which is the first day of Lunar New Year. The squirrel appeared earlier at 6:27 am on December 31, 2024, which is the 1st day of the 12th lunar month. The period from December 31st to January 28th is only 29 days, so the 12th lunar month of the Year of the Dragon is a short month, and the New Year's Eve to welcome the Year of the Snake only has the 29th day of Tet.
Thus, in 8 years from 2025 to 2032, the 30th of Tet will be continuously absent. This year's Binh Ngo Lunar New Year 2026 will also not have the 30th of Tet because it falls into this series of years.
However, even though December has 29 or 30 days, Vietnamese people also value the last day and the last moment of the Lunar New Year as a sacred moment of transition between the old year and the new year, called New Year's Eve.
Therefore, in the years of December, many people still have the habit of calling the 29th day of Tet the 30th day of Tet, because "30th day of Tet" has become a concept referring to the last day of the Lunar year.
