What is "intercalary day"

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.