My clients pay 49 weeks per year, full fees no matter what happens. I don't want to list snow days, sick days, etc. so I make it simple. My paid closed days are clearly listed so they can mark their calendars for the year and I let them know at the beginning of the year which 3 weeks I'll be taking for my unpaid vacations so they can plan those also.

In my first year I offered unpaid vacation days to clients but that was just insane. I had a difficult time budgeting and keeping track of everyone's days, so I changed it to payments all the time.