fortnight ......... means 2 weeks.
Perhaps not an idiom, but close enough. Derived from Old English language which literally means 14 nights.
Last week I was talking with an American friend here in Buriram who was puzzled when I said "fortnight"
When I explained "fortnight", he said "oh .... you mean biweekly."
This intrigued me, as being an Aussie, I've never before heard anyone refer to 2 weeks as "biweekly".
Just another example of cultural and national difference when using English language.