An event of some sort that happens twice a year.
If an event happens twice a year it is called 'Biannual.' :-) P.S - Don't get confused with 'Biennial"
Bi-weekly means twice a week. A bi-weely event happens twice a week.
If something is biannual, it happens twice a year.
biennial and biannual, althou biannual could also mean twice a year