But I think only GRRM could answer this for sure. I am trying to think if I have seen a reference to 'week' in the books. It seems much of the GoT world is like medieval western Europe, except on a totally different world.
We also do not know how long the moon cycle is in that world, although there is talk of menstrual cycles being similar to moon cycles. That would pin a lunar period of 28 days, and one fourth of that, a 7 day week. That is, if the menstrual cycles there are 28 days as well (which they probably are)