Sentence:  The Roman Month its several days divides/ By reckoning backwards, Calends, <WSD>Nones</WSD>, and Ides. 
Meaning of the word marked with <WSD> tags:  The notional first-quarter day of a Roman month, occurring on the 7th day of the four original 31-day months (March, May, Quintilis or July, and October) and on the 5th day of all other months.