Source : Webster's Revised Unabridged Dictionary (1913)

Intercalary \In*ter"ca*la*ry\ (?; 277), a. [L. intercalaris,
   intercalarius: cf. F. intercalaire. See {Intercalate}.]
   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.

   2. Introduced or inserted among others; additional;
      supernumerary. ``Intercalary spines.'' --Owen.

            This intercalary line . . . is made the last of a
            triplet.                              --Beattie.

   {Intercalary day} (Med.), one on which no paroxysm of an
      intermittent disease occurs. --Mayne.

Source : WordNet®

intercalary
     adj : added or having an addition to make the calender year
           correspond to the solar year; "Feb. 29 is an
           intercalary day"; "leap year is an intercalary year"