Correct usage of bi and semi

You can dispel the confusion caused by bi and semi by remembering that bi means two and semi means half. 
Here’s what H.W. Fowler, the outraged pedant of abused English, says about bi:

“If it were not for bicentenary, which lacks a vernacular equivalent, there would be no reason why all the bi- hybrids should not be allowed to perish.” —H.W. Fowler, Fowler's Modern English Usage

However, if you must use bi and semi, here's the correct usage:

• semiweekly = twice a week
• biweekly = every two weeks 
• semimonthly = twice a month 
• bimonthly = every two months 
• semiannual = twice each year 
• biennial = every two years