intercalary

(ĭn-tûrˈkə-lĕrˌē, ĭnˌtər-kălˈə-rē)

1. a. Inserted in the calendar to make the calendar year correspond to the solar year. Used of a day or month.
   b. Having such a day or month inserted. Used of a year.
2. Inserted between other elements or parts; interpolated.