The first day of each month is called the new moon, and the last day is called darkness. The fifteenth, sixteenth or seventeenth day of the lunar calendar is called Wangwang, and the day after Wangwang is called Wangwang, so New Year's Eve of the lunar calendar is not the same day every year. The basis of the lunar calendar is the year of the tropic of Capricorn and the first month. The tropic year is the time required for the sun to pass through vernal equinox twice in a row, that is, 365.2422 days (365 days, 5 hours, 48 minutes and 46 seconds). "The first month of the lunar calendar" is the time between two consecutive new moons (new moons) or two full moons (full moons), that is, 29.5306 days (June 29th 12 hours, 44 minutes and 3 seconds). Because 12 "Wang Shuoyue" is only 354.3672 days, which is more than 10 days different from 365.2422 days in the tropic year, this difference adds up to a leap month. As early as the Spring and Autumn Period in 5600 BC, China calendar adopted the arrangement of seven leap months in 19 years.