Question: why do we say biweekly?
Answer: Biweekly and bimonthly can mean the same thing because of the prefix bi-, which here can mean “occurring every two” or “occurring twice in.” Therefore, biweekly can be “twice in a week” or “every other week.” Bimonthly can also mean "every other week" if it's twice in a month, or it can mean “every other month.”