Why should there be Indra in the 24 solar terms of Buddhism?

Twenty-four days is the "twenty-four days" of adding four gods on the basis of twenty days. The names of the twenty days all come from Hinduism (Brahmanism), specifically:

One Brahma, Two Indra, Three Heavenly Kings, Four Heavenly Kings, Five Heavenly Kings, Six Heavenly Kings, Seven King Kong Mysteries, Eight Days of Freedom or Mohan Defending, Nine Fats Dispelling, Ten Days of Debate, Eleven Heavenly Virtues, Twelve Heavenly Gods, Thirteen Heavenly Gods, Fourteen Bodhi Tree Gods, Fifteen Guizi Mothers, Sixteen Morri Gods, the Son of Heaven in the Seventeenth Palace and Twelve Heavenly Gods.

Among them, twenty days often appear as a group in statues and murals, and sometimes appear in land and water paintings hanging during the land and water law meeting. However, there are also many temples dedicated to the twenty-four celestial phenomena, such as Tongyuan Temple in Kunming, Shanhua Temple in Datong, Shanxi and Jingu Temple in Wutai Mountain.

In the art of Buddhist grottoes in China, images of the sky appeared very early. For example, in the mural of the Buddhist story of "Walking through the city at midnight" painted on the left wall of the main room of Cave 100 of Qizil Grottoes in Xinjiang, the images of Indra and the Four Heavenly Kings appeared. On both sides of the gate of the eighth cave of Yungang Grottoes, the head of Moan riding a cow and the pigeon riding a golden-winged bird are carved.