Provided that the doyo has 18 days, the Ox day comes around twice when the first Ox day comes within 6 days from the first day of the doyo, in other words, when the first day of the doyo falls on the day of the Monkey, Cock, Dog, Boar, Rat or Ox (based on the twelve signs of the Chinese zodiac).
18日間として考えると土用入りの日から6日以内に丑の日があると（すなわち土用入りの日が申から丑の間の場合）もう一度丑の日が巡って来る。