To begin with, the eighth day is the holy day, the birthday of Śākyamuni Buddha, who is your father. Thirty-two inexplicable phenomena appeared on that day. In the first place, all the plants blossomed and bore fruits. In the second place, various treasures gushed forth from the earth. Thirdly, water sprang out of the fields without rainfall. Fourthly, it became as bright as day during the night. In the fifth place, no voice of sorrow was heard throughout the entire world, and so on. Thus, they were all lucky omens. Since that day until now, for 2,230 years or so, it has been customary for us to choose the eighth day for a happy gathering.

Shijō Kingo-dono Gohenji, Response to Lord Shijō Kingo, Writings of Nichiren Shōnin, Followers I, Volume 6, Page 154