Ah, English how she are spoke. What zmjezhd said. A "centennial" is a hundred years, not a hundred times a year, so "bienniel" or "biannual" would be two years, not twice a year.

My favourite is "sesqui" - a sesquicentennial is one hundred and fifty years. I call myself "sesquilingual" because I speak good English and half-assed French.