卯の日 - the day of u

U refers to the fourth sign of the junishi calendar. The junishi are the twelve signs of the Chines zodiac that are used to refer to directions and time. They are also used to refer to years and dates in combination with the ten calendar signs called jikkan.

「卯」は十二支の第4番目を指す。方角・時刻を表すために用いるが、十干と組み合わせて年・日を表すこともある。