Why is the Spring Festival the first of the 24 solar terms?

Spring Festival is not in the 24 solar terms, it is the first day of the first month. The first day of the lunar new year.

Twenty-four solar terms are specific:

A slight cold in January was followed by a severe cold, and it rained in the spring of February;

The vernal equinox is in March and Qingming Grain Rain is in April.

Long summer and full summer in May, full summer solstice in June;

July is hot and slightly hot, and August is long summer, beginning of autumn;

In September, the Millennium meets the autumnal equinox, and the first frost and cold dew are all in October;

There is a light snow in 1 1 month in beginning of winter, and a heavy snow in winter solstice to welcome the New Year.