Biannual
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.
semimonthly
Bi-weekly means twice a week. A bi-weely event happens twice a week.
it means something happened twice in a month!
Biscuit, from the Old French 'bi' twice, 'cuit' cooked.
Unexpected?
anachronism or apocrypha
Quinquennial = something that happens every 5 years. Lustral = also something that happens every 5 years or we could invent: semi-Decennial: where semi is half and Deccennial happens every 10 years.
occurance means "how often something happens or occurs"
acquire
Twice-baked potatoes The word 'biscuit' literally means baked twice.