Biweekly has two conflicting meanings. One sense of the word refers to an event that occurs every two weeks, while the other sense refers to an event that occurs twice in a given week. Both senses have been in use for over a century. The term fortnightly can be used less ambiguously for the first sense, with a two week interval equivalent to one fortnight. Semi-weekly, similarly, can be used for the second sense; every half week or twice a week.
There are fifty-two weeks in a year of twelve months, making for a possible 26 (or 104) biweekly events in a year. (This is a greater number than if such events were held twice a month, because most months have more than four weeks (28 days). However, biweekly is often thought of incorrectly as twice a month. Twice a month is called "semimonthly" or "bimonthly", not biweekly.)