Why did the long summer change from March to April this year?

When you say March, you mean March in the lunar calendar. The lunar calendar is called the lunar calendar, which is based on the first month of the lunar calendar (the time when the phase of the moon changes by one cycle). The characteristics of the lunar calendar are: the length of months is based on astronomical phenomena, and the average of calendar months is roughly equal to the first month of the lunar calendar, with 30 days in the first month and 29 days in the second month of the lunar calendar. The length of a year is only an integer multiple of the calendar month, which has nothing to do with the tropic year, and the month has nothing to do with the four seasons, cold and heat. Festivals in China are based on the lunar calendar.

When you say April, this April is April in the official calendar. The trunk calendar is a calendar marked with 60 different heavenly stems and earthly branches, which is a unique solar calendar in China. Day, month, year and time are the basic elements of a calendar. The trunk calendar is mainly composed of four parts: trunk calendar year, trunk calendar month, trunk calendar day and trunk duration. It takes beginning of spring as the beginning of a year, and the older year is the year of tropic of cancer. It is divided into twelve months and has twenty-four solar terms, and each month contains two solar terms.

There are four solar terms: beginning of spring, Changchun, beginning of autumn and beginning of winter. Four solar terms represent four seasons, one in March. The day when the four solar terms are established is the beginning of the four seasons and the first month of the four seasons. So the long summer is the beginning of April.

Although the long summer is on March 18 of the lunar calendar, and the official calendar is the first day of April on this day, you said that March changed to April.

Note: There are no leap months in the trunk calendar. The dry calendar is related to the periodic motion of the earth around the sun, and its year and day are determined by the apparent motion of the sun, regardless of the moon phase. It reflects the climate and phenological changes of the whole year more stably and accurately than the lunar calendar. In the long summer, the sun's longitude is 45 degrees, the heat is coming, and thunderstorms are increasing.