intercalary

英 [ɪn'tɜːkəl(ə)rɪ; ,ɪntə'kæl(ə)rɪ] 美[ɪn'tɝkə,lɛri]

1. 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"