Aren’t these two words confusing? Bi— means two, but which one refers to twice a year and which one means every two years? Here’s the scoop:
BIANNUAL means something that happens twice in one year: We change our clocks biannually.
BIENNIAL means an act that occurs once in every two years: United States congress members are up for re-election biennially.
I think these definitions are ones you just have to memorize. If you can think of a helpful trick, let me know.