Why is the ninth month called September?
early 15c., "one-sided, biased," from Old French parcial (14c., Modern French partial), from Medieval Latin partialis "divisible, solitary, partial," from Latin pars (genitive partis) "part" (see part (n.)). Sense of "not whole, incomplete" is attested from late 14c. Related: Partially (mid-15c. as "incompletely").