noun plural.
The 15th day of March, May, July, or October or the 13th day of the other months in the ancient Roman calendar.
The Century Dictionary and Cyclopedia
In the ancient Roman calendar, the eighth day after the nones —that is, the 13th of January, February, April, June, August, September, November, and December, and the 15th of March, May, July, and October.
the GNU version of the Collaborative International Dictionary of English
noun plural.
The fifteenth day of March, May, July, and October, and the thirteenth day of the other months.