Biannual, Biennial & Perennial

Biannual means occurring twice a year; semiannual: “The Ladakh festival takes place on a biannual basis, once in February and then in August.”
Biennial means ccurring every two years: “Electing for the seat of mayor occurs on a biennial basis.”
Perennial means persistent over a period of several years; continuing without interruption: “The rains have been perennial in this part of the country over the last few years.”