Monday Michiru Sipiagin (maiden name Mariano, more commonly known as Monday Michiru / MONDAY 満ちる born August 19, 1963) is a Japanese American actress, singer, and songwriter whose music encompasses and fuses a wide variety of genres including jazz, dance, pop, and soul. She is arguably best known for being a pioneer of the acid jazz movement in Japan in the early 1990s yet she has created her own unique style of music that transcends traditional definitions of the aforementioned genres.

Monday was born in Tokyo, Japan to jazz pianist Toshiko Akiyoshi and her then husband, jazz saxophonist Charlie Mariano. At an early age, she expressed an interest in music; she studied modern dance and ballet from age eight, and the classical flute for eight years. It wasn't until she went to the Interlochen Arts Academy that she discovered her true passion of singing.