Why is the ninth month called September?
1560s, "explanation of a term," from Middle French notation and directly from Latin notationem (nominative notatio) "a marking, notation, designation; etymology; shorthand; explanation," noun of action from past participle stem of notare "to note" (see note (v.)). Meaning "note, annotation" is from 1580s. Meaning "system of representing numbers or quantities by signs or symbols" is attested from 1706. Related: Notational.