Let’s imagine it’s currently Wednesday the 1st. Does “next Saturday” mean Saturday the 4th (the next Saturday to occur) or Saturday the 11th (the Saturday of next week)?
It’s the same with “bi-weekly”. Does that mean twice every week, or once every 2 weeks?
There’s a bunch of ambiguous stuff in the English language. The only real solution is just avoid ambiguity in the first place by not using the phrase