Biannual means that which occurs twice in a year.
Biennial means that which occurs every two years.
1. The Dental College conducted biannual free dental checkups for the poor at the campus.
2. Since the workshop on basic fashion designing got a good response, the Institute decided to conduct it biannually rather than just once a year.
1. The flower show was a biennial event.
2. The IPL cricket tournament should be held biennially to retain interest.