"Dwywaith yr wythnos"
Translation:Twice a week
Sorry, but I can't help it. I've just realised after this while that 'wythnos' is made up of 'wyth = eight' and 'nos = night'...is this just a coincidence or not? If it is not, why eight instead of seven?
Because a Week includes the Sunday night before hand then all the nights following up until the following Monday morning.
Plus I think I'm right in saying the Celts, like the Jews, started counting new days (periods of 24 hours) at nighttime.
Also your explanation also holds for pythefnos (fortnight, two weeks) from pymtheg (fifteen) + nos. Please don't go around around saying dwy wythnos in Welsh. Pythefnos sounds much better!
The French also say "huit jours" (8 days) to mean a week, even though "semaine" would seem to have more to do with sept/seven (although they do also say "une huitaine" to mean a week or thereabouts!