Nones (Nonae) were known to be the days of the half moon which usually occur 8 days before the Ides. Ides occurred on the 15th day of March, May, July, and October, and the 13th day of the other months. They are thought to have been the days of the full moon.
The addition of January and February moved the months September, October, November, and December to later in the year so they no longer correspond with the original meaning of their names. In the Roman calendar, their original names in Latin mean theseventh, eighth, ninth, and tenth month.