"Yang Gong's Forbidden Day", also known as "Thirteen Forbidden Days in Yang Gong", is a legend that has spread all over the world and was formulated by Yang Junsong, a geomantic master in the Tang Dynasty. This ancient custom began on the 13th day of the first lunar month, and then it is called "Yang Gong Taboo Day" two days in advance every month, also known as "Yang Gong Thirteen Taboos", which is a Pepsi Taboo Day. In the folk, most people know that on the thirteenth day of the first month, they don't work or travel. In fact, it is mainly to commemorate Yang Gong, that is, Yang Jiye the Golden Knife.