Group 1 - The lunar calendar is a traditional Chinese calendar that balances the lunar cycle and the solar year, consisting of 12 months, with each month being either 29 or 30 days long [2] - A year with only 12 lunar months averages 354 days, which is about 11 days shorter than the solar year, leading to seasonal misalignment [2] - To correct this, the ancient Chinese devised the method of adding a leap month every two to three years, known as a leap month [2] Group 2 - The reason for the occurrence of a leap month in June is tied to the 24 solar terms, which are crucial for determining leap months [4][6] - Each lunar month must contain a solar term; if a month lacks a solar term, it is designated as a leap month from the previous month [6] - In the current lunar calendar, the month following June contains only one solar term, which is the beginning of autumn, thus making June a leap month [6] Group 3 - There are six instances of leap June in this century, occurring in the years 2017, 2025, 2036, 2055, 2074, and 2093 [11][12]