Quadrenial
twice-a-day or it can be called semi-diurnal which relates to such occurrences as tides.
weekly
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.
decennial Occurring every ten (10) years. Also tenth anniversary
Something that happens once a century.
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Fortnightly. or you could just say bi-weekly that's what everyone says....
A triennial event happens every three years.
Something that happens every 5 years
Annual.
Something that happens every six years is sexennial.
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.