"An ancient once said: 'In the first month of spring, flourishing virtue lies in wood, and one should sacrifice to the door with wood.'"
"They also said: 'In spring, yang energy emerges. Sacrifice at the door, for it is inner yang.'"
"The first month of spring is the first month of the season, as well as the first month of the year—the first lunar month. During this season, all things sprout, and every form of life that has lain dormant through winter begins to stir. Trees are the most conspicuous of all, which is why 'wood' is used to sacrifice to the door."
"So-called 'sacrifice to the door' means offering sacrifice to the 'door.' It is the first of the Five Sacrifices. Correspondingly, there is also a 'sacrifice to the gate,' held in the ninth month, 'to gather the stored treasures of Heaven and Earth.'"
"As for why sacrifice to the door and sacrifice to the gate are separate, it is because the ancients distinguished between gates and doors." Dictionaries explained, "A single leaf is called a door; two leaves are called a gate." Others said, "A door lies at the east of a hall chamber, while a gate stands at the boundary of a residence." Still others believed that "what lies outside is called a gate, while what lies inside is called a door"... In short, according to ancient rites, sacrifice to the door and sacrifice to the gate were separate. Our family follows the last interpretation..."
As Zheng Qing washed his hands, burned incense, and reverently inserted Willow Branches above the many "doors" of Qingqiu Mansion, Su Shijun held Poseidon and tirelessly explained the related ancient rites to her.
To be honest, Zheng Qing was hearing much of it for the first time as well. He pricked up