I went for simplicity.
One rate, per week, regardless of attendance.
(monthly bills+$200 retirement/6 slots, for me)
I give them 2 weeks vacation, per family, per year, unpaid. I do require them to give me a 30 day notice so I can I budget around them.
I do not charge them for my two weeks vacation each year. I have my calendar available through 2013, currently, so budgeting requires little to no effort.
I do take all National Holidays that fall during the week, paid. I get no argument for this since I meet them halfway with scheduled vacations.
It really has made life more peaceful.