dictionary.com
Usage Note: Bimonthly and biweekly mean ?once every two months? and ?once every two weeks.? For ?twice a month? and ?twice a week,? the words semimonthly and semiweekly should be used. Since there is a great deal of confusion over the distinction, a writer is well advised to substitute expressions like every two months or twice a month where possible. However, each noun form has only one sense in the publishing world. Thus, a bimonthly is published every two months, and a biweekly every two weeks.