The Collaborative International Dictionary of English v.0.48:

Calendar \Cal"en*dar\, n. [OE. kalender, calender, fr. L.
   kalendarium an interest or account book (cf. F. calendrier,
   OF. calendier) fr. L. calendue, kalendae, calends. See
   Calends.]
   1. An orderly arrangement of the division of time, adapted to
      the purposes of civil life, as years, months, weeks, and
      days; also, a register of the year with its divisions; an
      almanac.
      [1913 Webster]

   2. (Eccl.) A tabular statement of the dates of feasts,
      offices, saints' days, etc., esp. of those which are
      liable to change yearly according to the varying date of
      Easter.
      [1913 Webster]

   3. An orderly list or enumeration of persons, things, or
      events; a schedule; as, a calendar of state papers; a
      calendar of bills presented in a legislative assembly; a
      calendar of causes arranged for trial in court; a calendar
      of a college or an academy.
      [1913 Webster]

   Note: Shepherds of people had need know the calendars of
         tempests of state. --Bacon.
         [1913 Webster]

   Calendar clock, one that shows the days of the week and
      month.

   Calendar month. See under Month.

   French Republican calendar. See under Vend['e]miaire.

   Gregorian calendar, Julian calendar, Perpetual
   calendar. See under Gregorian, Julian, and Perpetual.
      [1913 Webster]

WordNet (r) 3.0 (2006):

calendar month
    n 1: one of the twelve divisions of the calendar year; "he paid
         the bill last month" [syn: calendar month, month]