Mary Day may refer to:
People Mary Day (dance teacher) (1910–2006), founder of The Washington Ballet Mary Anna Day (1852–1924), American botanist and librarian Mary E. Day, in the 2005 Supreme Court opinion Varian v. Delfino Mary L. Day (1836–?), American memoirist Mary Gage Day (1857–1935), American physician and medical writer Mary Louise Day (1968–2017), teenage girl who mysteriously disappeared from her home
Days Marian feast days