The Vietnamese calendar (Vietnamese : âm lịch; Hán-Nôm: 陰曆) is a lunisolar calendar that is mostly based on the lunisolar Chinese calendar. As Vietnam's official calendar has been the Gregorian calendar since 1954,  the Vietnamese calendar is used mainly to observe lunisolar holidays and commemorations, such as Tết Nguyên Đán and Tết Trung Thu.
After Vietnam regained independence following the third Chinese dominion of Viet-Nam, monarchs established their own calendars based on Chinese prototypes, and every subsequent dynasty had appointed officers to man and create the calendar to be used in the realm.  According to the Đại Việt sử lược historical chronicles, the Vietnamese rulers started building astronomical/astrological facilities in the capital Thăng Long (Chữ Hán: 昇龍; i.e. modern Hanoi) as early as 1029.  Beginning in 1324, the Chinese Yuan dynasty introduced the Thụ Thời (Chinese :授時; pinyin :shòu shí) calendar to the Vietnamese Trần dynasty. 
|Calendar name||Year in use||Notes|
|Thụ Thời (Chinese :授時; pinyin :shòu shí)||1324–1339||Introduced by the Chinese Yuan dynasty to the Vietnamese Trần dynasty.|
|Hiệp Kỷ (Chữ Hán: 協紀)||1339–1401||Probably a name change with no changes to calculation methods. |
|Thuận Thiên (Chữ Hán: 順天)||1401–1413||Hiệp Kỷ calendar abolished, with Thuận Thiên replacing it. There was no documentation on the difference between the two. |
|Đại Thống (Chinese :大統; pinyin :dà tǒng)||1413–1813||Introduced by the Chinese Ming dynasty in 1369, during the Fourth Chinese domination of Vietnam, the Ming administration in Vietnam used the Datong calendar. At the start of the Vietnamese Lê dynasty in 1428, the end of Chinese domination over Vietnam, there was no evidence that the calendar was changed. |
|Hiệp Kỷ (Chữ Hán: 協紀) —Shíxiàn (Chinese :時憲; pinyin :shí xiàn)||1813–1840||Hiệp Kỷ is not to be confused with its earlier namesake. It was essentially the Shíxiàn calendar introduced by the Chinese Qing dynasty to the Vietnamese Nguyễn dynasty. |
|Gregorian||early 19th century||The Gregorian calendar was introduced by the French, and was used in Vietnamese administrative offices at the same time as the Hiệp-kỷ calendar, which remained in use by the Vietnamese royal court. |
|Hiệp Kỷ (Chữ Hán: 協紀)||1841–1954||Beginning in 1841, Hiệp Kỷ began to differ from Shíxiàn due to longitudinal differences between Vietnam and China. |
Beginning in 1954, Vietnamese administrative offices officially used the Gregorian calendar, while the civilian populace continued to use a variety of local calendars derived from French, Chinese and Japanese sources, including the Hiệp Kỷ calendar.  On 8 August 1967, the North Vietnamese government issued a decree to change Vietnamese standard time from UTC+8 to UTC+7, as well as make the Gregorian calendar the sole official calendar, restricting lunisolar calendar use to holidays and commemorations. South Vietnam would later join this change at the end of the Vietnam War in 1975.
The Chinese calendar is based on astronomical observations and is therefore dependent on what is considered the local standard time. North Vietnam switched from UTC+8 to UTC+7 on 8 August 1967, with South Vietnam doing likewise in 1975 at the end of the Vietnam War. As a result of the shift, North and South Vietnam celebrated Tết 1968 on different days.  This effect would see the solstice falling on 21 December in Hanoi, while it was 22 December for Beijing.
As the 5th month of the Chinese calendar must contain the summer solstice, it is not the month from 24 May 1986 to 21 June 1986 as per the Vietnamese calendar, but rather the one from 22 June to 20 July 1986. The two calendars agreed again after a leap month lasting from 22 June to 20 July of that year was inserted into the Vietnamese calendar.
As the 3th month of the Chinese calendar must contain the corn rain, it is not the month from 22 March to 19 April 2008 as per the Vietnamese calendar, but rather the one from 20 April to 19 May 2008. The two calendars agreed again after a leap month lasting from 20 April to 19 May of the year was inserted into the Vietnamese calendar
In the Vietnamese zodiac, the cat replaces the Rabbit in the Chinese zodiac. So, a child born in the Chinese year of the Rabbit will be born in the Vietnamese year of the Cat (mẹo/mão). The Vietnamese zodiac uses the same animals as the Chinese zodiac for the remaining 11 years, though the Ox of the Chinese zodiac is usually considered to be a water buffalo (sửu/trâu) in the Vietnamese zodiac.
The traditional Chinese calendar is a lunisolar calendar which identifies years, months, and days according to astronomical phenomena. In China, it is defined by the Chinese national standard GB/T 33661–2017, "Calculation and Promulgation of the Chinese Calendar", issued by the Standardization Administration of China on May 12, 2017. Traditional Chinese calendar also known as these five titles: Nongli Calendar, Jiuli Calendar, Laoli Calendar, Zhongli Calendar, Huali Calendar
A leap year is a calendar year that contains an additional day added to keep the calendar year synchronized with the astronomical year or seasonal year. Because astronomical events and seasons do not repeat in a whole number of days, calendars that have a constant number of days in each year will unavoidably drift over time with respect to the event that the year is supposed to track, such as seasons. By inserting ("intercalating") an additional day or month into some years, the drift between a civilization's dating system and the physical properties of the Solar System can be corrected. A year that is not a leap year is a common year.
A lunisolar calendar is a calendar in many cultures, combining lunar calendars and solar calendars. The date of Lunisolar calendars therefore indicates both the Moon phase and the time of the solar year, that is the position of the Sun in the Earth's sky. If the sidereal year is used instead of the solar year, then the calendar will predict the constellation near which the full moon may occur. As with all calendars which divide the year into months there is an additional requirement that the year have a whole number of months. In some case ordinary years consist of twelve months but every second or third year is an embolismic year, which adds a thirteenth intercalary, embolismic, or leap month.
A solar calendar is a calendar whose dates indicate the season or almost equivalently the apparent position of the Sun relative to the stars. The Gregorian calendar, widely accepted as a standard in the world, is an example of a solar calendar. The main other types of calendar are lunar calendar and lunisolar calendar, whose months correspond to cycles of Moon phases. The months of the Gregorian calendar do not correspond to cycles of the Moon phase.
Chinese astrology is based on traditional Chinese astronomy and the Chinese calendar. Chinese astrology flourished during the Han Dynasty.
In the Gregorian calendar, New Year's Day is the first day of the year; 1 January. Whilst most solar calendars begin the year regularly at or near the northern winter solstice, cultures that observe a lunisolar or lunar calendar celebrate their Lunar New Year at less fixed points relative to the solar year.
Lunar New Year is the first new moon of a lunar calendar or lunisolar calendar year, whose months are moon cycles. The event is celebrated by numerous cultures in various ways at diverse dates.
The Mid-Autumn Festival, also known as the Moon Festival or Mooncake Festival, is a traditional festival celebrated in Chinese culture. Similar holidays are celebrated in Japan, Korea, Vietnam, and other countries in East and Southeast Asia.
Calendar reform or calendrical reform is any significant revision of a calendar system. The term sometimes is used instead for a proposal to switch to a different calendar design.
The Buddhist calendar is a set of lunisolar calendars primarily used in Tibet, Cambodia, Laos, Myanmar, India, Sri Lanka, and Thailand as well as in Malaysia, Singapore and Vietnam by Chinese populations for religious or official occasions. While the calendars share a common lineage, they also have minor but important variations such as intercalation schedules, month names and numbering, use of cycles, etc. In Thailand, the name Buddhist Era is a year numbering system shared by the traditional Thai lunar calendar and by the Thai solar calendar.
A solar term is any of twenty-four periods in traditional Chinese lunisolar calendars that matches a particular astronomical event or signifies some natural phenomenon. The points are spaced 15° apart along the ecliptic and are used by lunisolar calendars to stay synchronized with the seasons, which is crucial for agrarian societies. The solar terms are also used to calculate intercalary months; which month is repeated depends on the position of the sun at the time.
The Trần dynasty, (Vietnamese: Nhà Trần, chữ Nôm: 茹陳）also known as the House of Trần, was a Vietnamese dynasty that ruled over Đại Việt from 1225 to 1400. The dynasty was founded when emperor Trần Thái Tông ascended to the throne after his uncle Trần Thủ Độ orchestrated the overthrow of the Lý dynasty. The Trần dynasty defeated two Mongol invasions, most notably during the decisive Battle of Bạch Đằng River in 1288. The final emperor of the dynasty was Thiếu Đế, who was forced to abdicate the throne in 1400, at the age of five years old in favor of his maternal grandfather, Hồ Quý Ly.
Aries (♈︎) is the first astrological sign in the zodiac, spanning the first 30 degrees of celestial longitude, and originates from the Aries constellation. Under the tropical zodiac, the Sun transits this sign from approximately March 21 to April 19 each year. This time duration is exactly the first month of the Solar Hijri calendar.
Trần Thái Tông, personal name Trần Cảnh or Trần Nhật Cảnh, temple name Thái Tông, was the first emperor of the Trần dynasty, reigned Đại Việt for 33 years (1226–58), being Retired Emperor for 19 years. He reigned during the first Mongol invasion of Vietnam before eventually abdicating in favor of his son Trần Hoảng in 1258.
The Burmese calendar is a lunisolar calendar in which the months are based on lunar months and years are based on sidereal years. The calendar is largely based on an older version of the Hindu calendar, though unlike the Indian systems, it employs a version of the Metonic cycle. The calendar therefore has to reconcile the sidereal years of the Hindu calendar with the Metonic cycle's near tropical years by adding intercalary months and days at irregular intervals.
The Chinese zodiac is a traditional classification scheme based on the Chinese calendar that assigns an animal and its reputed attributes to each year in a repeating twelve-year cycle. Originating from China, the zodiac and its variations remain popular in many East Asian and Southeast Asian countries, such as Japan, South Korea, Vietnam, Singapore, Nepal, Bhutan, Cambodia, and Thailand.
Prince of Bảo Nghĩa Trần Bình Trọng (1259–1285) was a general of the Trần Dynasty well known for his heroic action in the war of Dai Viet against the second Mongol invasion. After the capture of Trần Bình Trọng by the army of the Yuan Dynasty, Kublai Khan's prince Toghan attempted to convince the Đại Việt general to surrender by telling him about the situation of the Trần Dynasty, saying that Trần Bình Trọng would get a minister's post in China. However, the general rejected his proposition, and thus, Toghan had him executed. Today, he is widely known as an example of patriotism in Vietnam. The story about Marquis Bảo Nghĩa is taught in Vietnamese textbooks and many places in Vietnam are named in honour of this national hero.
National Day is a national holiday in Vietnam observed on 2 September, commemorating President Hồ Chí Minh reading the Declarations of independence of Vietnam at Ba Đình Square in Hanoi on 2 September 1945. It is the country's National Day.
The House of Nguyễn Phúc, also known as the House of Nguyễn Phước, was a ruling family of Vietnam. It ruled from the city of Huế in central Vietnam beginning in 1636. As the Nguyễn lords, they often fought with the Trịnh lords, who were based in Hanoi. They were overthrown by the Tây Sơn dynasty in 1777 but restored in 1780.
The seals of the Nguyễn dynasty can refer to a collection of seals specifically made for the emperors of the Nguyễn dynasty, who reigned over Vietnam between the years 1802 and 1945, or to seals produced during this period in Vietnamese history in general.