"No, I do not help in the shop every week."
Translation:Nac ydw, dw i ddim yn helpu yn y siop bob wythnos.
Fortnight (fourteen nights) is 'Pythefnos' in Welsh, which means fifteen nights.
I don't know an official explanation but there is a way that this makes sense. If you start at midnight one night and count for 7x 24 hours you finish at the eighth midnight, including the first one, thus the length of one week means the experience of 8 nights.
Ditto the experience of a fortnight