Ides

noun

1. (often capitalized) 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.

   "The third day before the ides of March is March 13th; the third ides of August is August 11th; and the third of the ides of November is November 11th."