intercalary

attrib. Adj. 1 a (of a day or a month) inserted in the calendar to harmonize it with the solar year. B (of a year) having such an addition. 2 interpolated. [latin calo proclaim]

a. 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.