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.
Oh, dude, that's like some riddle inception right there. So, technically, the answer would be the letter "e." It appears once in the word "year," twice in "month," four times in "week," and six times in "day." Mind blown, right?
The letter 'E'
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