early 15c., in reference to the Roman calendar, "ninth day (by inclusive reckoning) before the ides of each month" (7th of March, May, July, October, 5th of other months), from Latin nonæ (accusative nonas), fem. plural of nonus "ninth." Ecclesiastical sense of "daily office said originally at the ninth hour of the day" is from 1709; originally fixed at ninth hour from sunrise, hence about 3 p.m. (now usually somewhat earlier), from Latin nona (hora) "ninth (hour)," from fem. plural of nonus "ninth," contracted from *novenos, from novem "nine" (see nine). Also used in a sense of "midday" (see noon).
For for the nones, see nonce.