Why is it almost a decade before Tet comes around?

Social networks are buzzing with interesting information that after December 30th of the year of the Cat, we will have to wait another 9 years, that is, 2033, to "see" the 30th of Tet again. In the years in between, we welcome the New Year's Eve on the night of December 29th.

This information surprised many people and made them feel very excited, but many people were also skeptical, quickly checked the calendar and were delighted to find that it was true.

Why is it almost a decade before Tet comes around?

New Year's Eve will fall on the 30th of Tet if it is a full year, but for short years, this moment comes when the 29th of Tet ends. The year of Quy Mao 2023 ends on the 30th of December, which means that on the occasion of the Lunar New Year of Giap Thin 2024, we will still have the 30th of Tet.

However, there is a truth that few people know that after that, we will have to wait another 9 years to enjoy the feeling of the real 30th of Tet. From 2024 onwards, we will only be able to celebrate New Year's Eve on the 29th of Tet because for 8 consecutive years (from 2025 - 2032), December only has 29 days.

Why is it almost a decade before the 30th of Tet? This is related to the algorithm for calculating the lunar calendar. Unlike the solar calendar which is based on the cycle of the Earth's movement around the Sun (rounded to 365 days, each month has 30 or 31 days), in the lunar calendar, the number of days in a month is calculated based on the cycle of the Moon in relation to the Earth and the Sun.

The moment these three celestial bodies are in a straight line in the order Earth - Moon - Sun, the observer standing on Earth can no longer see the Moon, that is the 1st day (new moon day).

The moment when these three celestial bodies are in a straight line in the order of Moon - Earth - Sun, that is the time of the full moon. The full moon day (15th lunar month) may not necessarily fall on the full moon day, but the first day is always the New Moon day.

The average cycle of the Moon from full to waning is 29.53 days. Meanwhile, the number of days in each month must be an even number, which is why the Lunar calendar has extra and short months.

The phenomenon of 8 consecutive years of December being short - only 29 days - like the period 2025 - 2032 as mentioned above is just a coincidence. This phenomenon is little known and very interesting, but not necessarily too rare. For example, from the year of Binh Than (2016) to the year of Canh Ty (2020), there were 5 consecutive years of December being full.