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triannual
Bi-monthly
If this is a riddle the answer is the letter 'e'. 2 times in wEEk. Once in yEar. there are no 'E's in day.
The letter 'E'
Because that happens to be the number of times the Earth spins during the time it takes to circle the sun once in its orbit.
triannual
Annual.
weekly
An annual event
quarterly
makes no sense
If something is biannual, it happens twice a year.
An annual event starts with A and happens once a year.
bimonthly
Something that happens once a year.
Biannual or semiannual.
season