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During the Spring and Autumn Period and the Warring States Period, there were three calendar systems used in different regions: the summer calendar, the Yin calendar and the weekly calendar, the main difference is that the month of the first year is different. The twelve months of a year are the sub-month, the ugly month, the yin month, the 卯月, the tatsu month, the 巳 month, the noon moon, the unmoved month, the Shen month, the unitary month, the 戌月, and the Hai month. Among them, the month where the winter solstice is located (the month of mid-winter) is the sub-month, the month where the great cold is located (the month of the season winter) is the ugly month, the month where the rain is located (the month of Mengchun) is the yin month, and so on.
The summer calendar uses the yin month as the first month (the same as the current lunar month), the Yin calendar with the ugly month (equivalent to the current lunar month of December) as the first month, and the weekly calendar with the sub-month (equivalent to the november of the present lunar calendar) as the first month. The traditional calendar we use today is the summer calendar, also known as the lunar calendar and the lunar calendar. This calendar consists of twenty-four solar terms to guide agricultural activities, a strict synodic cycle to determine the month, and a leap month to make the average length of the year similar to that of the regression year, which has the nature of both a lunar month and a solar year, and is therefore essentially a luni-solar calendar.
The lunar calendar takes the date of the sun and moon conjunction (the sun and the moon are equal to the yellow longitude) as the first day of the month, that is, the first day of the first month. Synodic months are about 29.53059 days on average, so some months are 30 days, called the moon larger; some months are 29th, called the moon small. The date of the beginning of the month is determined by extrapolation based on the position of the Sun and the Moon, rather than being arranged mechanically. The lunar calendar has 12 months as a year, a total of 354 or 355 days, which is about 11 days different from the return year, so every three years there is a leap month, and then two years to insert a leap month, with an average of seven leap months in nineteen years.
In which month to place a leap month, it is necessary to determine the specific twenty-four solar terms that are closely related to the season and climate. A regression year is divided into twenty-four solar terms, and if the twenty-four solar terms are discharged from Lichun to The Great Cold, then the odd number is called the solar terms, and the even number is called the middle qi. The name of the lunar month is determined by "medium gas", such as the month with "rain" is called the first month, and the month with the "spring equinox" is called February. Twelve mid-airs are used to characterize the twelve months of a year, and the average between the two mid-air is 365.2422÷ 12=30.4368 days, then 30 or 31 days, which is nearly one day more than a synodic month.
The Earth's orbit around the Sun is elliptical, moving fast at perihelion (vernal equinox, autumn equinox), and the distance between the two solar terms can be up to 31.45 days. The average synodic month of the lunar calendar is only 29.5306 days, the lunar month is 29 days, and the big month is 30 days. This leads to the situation in which some months (usually small months) begin with the previous mid-air already passed, and at the end the next mid-air has not yet come. So this lunar month without neutrality is called the leap month of the previous month. For example, in 2001 (year of the snake), Xiao man is on May 21 (the 29th day of the fourth month of the lunar calendar), and the first day of the next lunar month is May 23, and the small full has been missed, but this lunar month ends on June 20, and the summer solstice is June 21, then this lunar month has no mid-breath, and after the fourth month of the lunar calendar, it is called leap April. According to such leap rules, leap July 2006, May 2009, Leap April 2012, Leap September 2014, Leap June 2017, Leap April 2020, and Leap February 2023.
This year's leap April has only one mango festival, not a mid-air. This "leap April" begins on May 23 and ends on June 20 in the gregorian calendar, with a month length of 29 days. Using this method to set the leap month can make the calendar month and the solar term not too far apart.
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Source: Xinhua Encyclopedia of China Database