A group of 15 days is called?

Many languages have a term for "half a lunar month" but English doesn't (or doesn't any more). I have Welsh heritage though and, to them, the literal definition of week is eight days and while we have a fortnight in Engand (fourteen days), they have pythefnos which literally means fifteen days. Nos is day and pythef is a contraction of pymtheg (which means fifteen). In the same way, fortnight is a contraction of fourteenth-night.