The Collaborative International Dictionary of English v.0.48:

Intercalary \In*ter"ca*la*ry\ (?; 277), a. [L. intercalaris,
   intercalarius: cf. F. intercalaire. See Intercalate.]
   [1913 Webster]
   1. (Chron.) Inserted or introduced among others in the
      calendar; as, an intercalary month, day, etc.; -- now
      applied particularly to the odd day (Feb. 29) inserted in
      the calendar of leap year. See Bissextile, n.
      [1913 Webster]

   2. Introduced or inserted among others; additional;
      supernumerary. "Intercalary spines." --Owen.
      [1913 Webster]

            This intercalary line . . . is made the last of a
            triplet.                              --Beattie.
      [1913 Webster]

   Intercalary day (Med.), one on which no paroxysm of an
      intermittent disease occurs. --Mayne.
      [1913 Webster]

WordNet (r) 3.0 (2006):

intercalary
    adj 1: having a day or month inserted to make the calendar year
           correspond to the solar year: "Feb. 29 is an intercalary
           day"; "a leap year is an intercalary year"