The Nuttall Encyclopedia
The name given in the Roman calendar to certain days that divide the month; in March, May, July, and October they fall on the 15th, in the rest on the 13th.
Wood, James, ed. Entry for 'Ides'. The Nuttall Encyclopedia. https://www.studylight.org/encyclopedias/eng/nut/i/ides.html. Frederick Warne & Co Ltd. London. 1900.