That would depend on where you live and what you class as an event. To me an event is a special occasion like a dance, exhibition etc, not going to church,,so.........think of what it means to you,
weekly is every week. bi-weekly is every 2 weeks
Bi-weekly means twice a week. A bi-weely event happens twice a week.
If something happens twice a month it is called biweekly because it happens every other week and a month has 4 weeks. there is also bimonthly which is every other month but not really a direct word for something that happens twice a month.
It is a triennial event. A triennial event happens three times a year.
Change.
A triennial event happens every three years.
weekly
yes i'm positive it happens every year
Something that happens every six years is sexennial.
nothing
bidecadal
Semicentennial