Sentence:  The Roman Month its several days divides/ By reckoning backwards, Calends, Nones, and <WSD>Ides</WSD>. 
Meaning of the word marked with <WSD> tags:  The notional full-moon day of a Roman month, occurring on the 15th day of the four original 31-day months (March, May, Quintilis or July, and October) and on the 13th day of all other months.