Is it the first day of the lunar calendar or the twenty-four solar terms?

The first lunar calendar. Twenty-four solar terms began in the Western Zhou Dynasty, basically shaped in the Spring and Autumn Period and the Warring States Period, and finally completed in the Western Han Dynasty. As far as the existing literature is concerned, the complete "twenty-four solar terms" first appeared in the Taoist classic Huainanzi in the Han Dynasty. As far as the calendar is concerned, the taichu calendar written by Deng Ping in the Western Han Dynasty set 24 solar terms in the calendar for the first time, and defined the astronomical position of the 24 solar terms.