Why does leap in the first month rarely happen?

Because leap month is related to the 24 solar terms, which are divided into solar terms and neutral terms, and are arranged alternately according to neutral terms and solar terms from winter to Sunday. Since the average interval between every two solar terms in a year is about 30.5 days, while the average solar term in each month in the lunar calendar is about 29.5 days, the solar term in each month will be delayed by one or two days from last month. In this way, there will eventually be a month with only solar terms but no neutrality, and this month will be designated as an extra leap month in a year. The so-called "seven leaps in nineteen years" is also determined in this way. Because the 24 solar terms are evenly distributed in the orbit around the earth, setting a leap month in this way can basically ensure that the average length of the lunar year is consistent with the tropic year.

Knowing the setting method of leap month, we can know the reason why leap month does not appear in the first month. Around the winter solstice, the earth moves at perihelion, and the revolution speed is relatively fast, while the 24 solar terms are evenly distributed in the earth's orbit, with one solar term every 15 degrees. At this time, it takes less time for the earth to make a circle around 15 degrees (15 degrees between every two solar terms), and it does not need 15 days. At this time, the interval between the two solar terms is only 29. In summer, when the earth moves near the perihelion, the revolution speed is slow, and the time required to move to 15 degrees reaches 16 days, so the solar term is delayed quickly at this time, and leap is more likely to occur.

Therefore, the leap in the twelfth lunar month and the leap in the first month are not uncommon, but relatively rare.