This is one of the more confusing usages in the language. It can mean either, although the more popular usage means every two weeks. In the US, our 1976 bicentennial was a celebration of 200 years, not 50.

From dictionary.com:

bi·week·ly adj.
Happening every two weeks.
Happening twice a week; semiweekly.

n. pl. bi·week·lies
A publication issued every two weeks.

adv.
Every two weeks.
Twice a week; semiweekly. See Usage Note at bi-1.


Source: The American Heritage® Dictionary of the English Language, Fourth Edition
Copyright © 2000 by Houghton Mifflin Company.
Published by Houghton Mifflin Company. All rights reserved


>> I thought, that bi-weekly means, what you receive your pay check every second week.
>
>No, it means twice per week. As in bi-centenial (fifty years)
>No big deal, best,
>Viv