Before adopting the Gregorian calendar, some East Asian countries (China, Japan, Korea and Vietnam) used the type of calendar which is often known as the Chinese Lunar Calendar. Despite being called “Lunar”, the calendar is in nature luni-solar. It is devised based on both the astronomical observation of the Moon and the changes of the seasons in a year which is the influence of the movement of the Earth around the Sun.

A year in the Chinese Lunar Calendar has 12 months in which a “full” month has 30 days and the
“not full” month has 29 days. One year represents a complete seasonal cycle. There are special years (often translated into English as “leap year”) that have 13 months. A period of 19 years has 7 leap years. The leap years are determined based on the prediction of the change of the season. They are often years that have unusual climate such as a long summer or winter. A full moon day is always the 15th day of the month. Like Gregorian calendar, a day in Chinese Lunar Calendar starts (roughly) at midnight.

A year of the Chinese Lunar Calendar is not numbered. Instead it is named with one element from the ten Heavenly Stems and the twelve Earthly Branches (the Earthly Branches are more commonly known under the name “Chinese Zodiac”). The ten Heavenly Stems in correct order are “jia”, “yi”, “bing”, “ding”, “wu”, “ji”, “geng”, “xin”, “ren” and “gui”, each of which is the Yang or Yin side of one of the five basic elements in Chinese cosmopolitan view (wood, fire, earth, metal and water). The twelve Earthly Branches in correct order are “zi” (the Rat), “chou” (the Ox), “yin” (the Tiger), “mao” (the Rabbit), “chen” (the Dragon), “si” (the Snake), “wu” (the Horse), “wei” (the Sheep), “shen” (the Monkey), “you” (the Rooster), “xu” (the Dog) and “hai” (the Pig). In the Vietnamese version of the Calendar, the Cat replaces the Rabbit and the Goat is in the Sheep’s place. Because the smallest common product of 10 (Heavenly Stems) and 12 (Earthly Branches) is 60, the name of the Lunar Calendar year repeats every 60 year. Because of the order of the elements must be observed, some combinations are never actually used such as “Jia Chou” (“jia” and the Ox).

The twelve Earthly Branches are also used to named the hours of the day. Thus a day in Chinese Lunar Calendar has 12 hours instead of 24. The hour of the Rat is from 11 p.m. to 1 a.m., the Ox 1 a.m. - 3 a.m., the Tiger 3 a.m. - 5 a.m., the Rabbit 5 a.m. - 7 a.m., the Dragon 7 a.m. - 9 a.m., the Snake 9 a.m. - 11 a.m., the Horse 11 a.m. - 1 p.m., the Sheep 1 p.m. - 3 p.m., the Monkey 3 p.m. - 5 p.m., the Rooster 5 p.m. - 7 p.m., the Dog 7 p.m. - 9 p.m. and the Pig 9 p.m. - 11 p.m.. In the field of horoscope, the months are also called after Earthly Branches. It is believed that each animal of the Earthly Branches symbolizes certain characteristics of nature and human. For example, the year of the Dragon often has more rain and consequently floods and storms than other years. Taking that into account together with the element of the Heavenly Stems, an experienced user of the Chinese Lunar Calendar can roughly predict the course of weather in the year, which is important in agricultural countries.

Also for agricultural purpose, there are 24 special days marked in the Lunar Calendar called “jiaki”. They are the days which mark the seasonal change and are named after the typical characteristic of that period. They are “Beginning of Spring”, “Slight Rain”, “Worms appearing”, “Mid Spring”, “Clear Sky”, “Shower”, “Beginning of Summer”, “Small Flood”, “Rising Star”, “Mid Summer”, “Slight Heat”, “Strong Heat”, “Beginning of Autumn”, “Rain that Lasts Long”, “Slight Sunshine”, “Mid Autumn”, “Cool Weather”, “Fog Appearing”, “Beginning of Winter”, “Snow Appearing”, “Strong Snow”, “Mid Winter”, “Slight Cold” and “Strong Cold”.

Nowadays, the Chinese Lunar Calendar is mostly used to calculate traditional holidays and festivals. It still play an important role in the countryside where agriculture still depends much on nature. Another use of the Chinese Lunar Calendar is Chinese horoscope where people calculate the good day for business, marriage, travel, etc based on the characteristic of the time describe in the Calendar.