> I understand that counting from the first Sunday to the last Sunday gives fifteen days, but why would we arbitrarily eliminate the Saturday. That explanation seems a good example of begging the question.
And if we count the actual number of days we are not at work, it is sixteen.


But work periods as short as 5 days are a modern and arbitrary superimposition compared to the age of this language. '15 days' surely, as Alex describes, just takes any given day and takes until its third recurrence. It does seem a little odd to count the ground on which you're standing, but then we all had the benefit of modern maths instructione, two!