Source : Webster's Revised Unabridged Dictionary (1913)

Roman calendar \Roman calendar\
   The calendar of the ancient Romans, from which our modern
   calendars are derived. It is said to have consisted
   originally of ten months, Martius, Aprilis, Maius, Junius,
   Quintilis, Sextilis, September, October, November, and
   December, having a total of 304 days. Numa added two months,
   Januarius at the beginning of the year, and Februarius at the
   end, making in all 355 days. He also ordered an intercalary
   month, Mercedinus, to be inserted every second year. Later
   the order of the months was changed so that January should
   come before February. Through abuse of power by the pontiffs
   to whose care it was committed, this calendar fell into
   confusion. It was replaced by the Julian calendar. In
   designating the days of the month, the Romans reckoned
   backward from three fixed points, the calends, the nones, and
   the ides. The calends were always the first day of the month.
   The ides fell on the 15th in March, May, July (Quintilis),
   and October, and on the 13th in other months. The nones came
   on the eighth day (the ninth, counting the ides) before the
   ides. Thus, Jan. 13 was called the ides of January, Jan. 12,
   the day before the ides, and Jan. 11, the third day before
   the ides (since the ides count as one), while Jan. 14 was the
   19th day before the calends of February.

Source : WordNet®

Roman calendar
     n : the lunar calender in use in ancient Rome; replaced by the
         Julian calendar in 46 BC