An old story goes that once there were four single men, leading very boring lives. None of them were married, or had lovers, or did anything exciting. They just sat around all day and played Mahjong.
One day they played Mahjong from 11 in the morning until 11 at night. During the game, no matter who won, the winning card was always the 'four columns' card （the card shows four independent, parallel columns in two lines）。 Even more of a coincidence, it was Nov 11, or 11/11. In order to commemorate the day, they nicknamed it "Singles Day".