“The sky is divided into 24 segments or jiéqì (节气) based on the seasons of the year. The earliest calendars assumed that the motion of the sun was constant and divided the year into 24 segments with equal numbers of days. This method is called píngqì (平气). Because the motion of the sun is not consistent, this was found to be inaccurate. The calendar then changed to a method where the ecliptic (the path of the sun as seen from earth) was divided into 24 equal parts of 15 degrees. This method is called dìngqì (定气). Calendars from the Warring States Period through the Ming Dynasty used the pingqi method in their designs and only changed to the more accurate dingqi method during the Qing Dynasty.” (source: hua.umf.maine.edu)
The 24 jiéqì 节气 consist of the succession of 立春 Lì chūn "Beginning of spring" (Spring festival), 雨水 Yǔ shuĭ "Rain water", 惊蛰 Jīng zhé "Waking of insects", 春分 Chūn fēn "Spring equinox" (March 21), 清明 Qīng míng "Pure brightness", etc.