Sat, Sun, Mon, Tue, Wed, Th, Fr, Sat, Sun, Mon, Tue, Wed, Th, Fr, Sat, Sun.

A total of seize jours, not quinze.



This can only work if you are talking about the working week, ma belle bel (I would have called you "bon bel" but for the possible offence you might take with ref. to the laughing cow!)
The whole point about the term "fortnight" is that it can be used about a period that starts on any day, related to any activity, as in, "I will see you again a fortnight today." Are you telling me that "quinze jours" can only be used to denote the period between one week-end and another?

If so, then you are right and I withdraw with a graceful bow, but I had always believed that the two phrases were equivalent.