They run from the day you start your W/W to the day it ends. Some people run their weeks Sun-Sat and others from the day they weigh to the day before they weigh and still others from the day after they weigh to the day they weigh.
So whatever it is when you start your journal week that is when they start.
I do it from the day I start my new daily tracker/journal thing. Which for me is Saturday to Saturday. But I don't think it really matters, as long as it's the same each week.