Why is it called Yang Gongji on the 13th day of the first month?

Thirteen taboos in folk calendars (you wouldn't believe it without scientific basis).

Thirteen monsters are thirteen bad days set by Yang Junsong for quietness and immobility. These thirteen days can't be chosen as opening a business, breaking ground, decorating and building a house, getting married, moving into a house, entering a job, traveling, having children, burying and building a grave, signing a contract and so on.

This taboo legend was adapted from fairy tales and then compiled into the Yellow Calendar to show the daily taboos of ancient people.

Legend related to extended data:

An old man surnamed Yang has thirteen sons, each of whom is as rich as a wolf. It can be said that some people need money, and Yang Gong is a bit eccentric by nature.

One day, he was very happy when chatting with someone. He boasted, "I just haven't given birth to a son for a month. I can still have a son to celebrate the New Year."

Who would have thought that Yang Laotou's wild words were heard by passing immortals and reported to heaven. The jade emperor was so angry that he ordered to accept a son every month. Since the 13th day of the first month, Yang Gong's son has to walk once a month, and he has to walk for two days every month. On the thirteenth day of the first month, the eleventh day of February and the ninth day of March, push it down in turn.

What's more, this year happens to be leap July. Therefore, Yang Gong's two sons were accepted in July, which were July 1 day and July 29th. So this year is 13 months, and none of Yang Gong's 13 sons left. By the end of the year, naturally no one will spend the New Year with Yang Gong. Later, people called the day when their son died Yang, thinking that these days were too poisonous, and we should try to avoid going out to do things.