Why did ancient people shave their heads to see the sun?

There is a saying in the ancient book "The Book of Filial Piety": When the body is skinned, parents dare not damage it, and filial piety begins. . .

Therefore, it is generally not easy to shave and get a haircut, but watching shaving days is generally for good luck and taboo.

I remember when I was a child, my elders would say that my uncle died in the first month. I wonder if it was handed down from ancient times.