It looks like this issue really comes down to personal preference (on behalf of the provider and parents) and what the "norm" is in your area.
I take off stat holidays, 10 calendar days vacation time and up to 4 personal days in a year. If my rates were on the higher end of the spectrum, then I may not be able to get away with this. But I am priced completely within normal range for my area. Full time parents pay a flat monthly fee, each and every month. As I have mentioned before, this secures them a spot in the dayhome for that month, under the agreed upon timeframes regarding drop off and pick up. I don't really charge daily or hourly fees in that sense. My part timers pay a daily rate (more than what the full time daily rate would be). They pay for the days they use, so they don't pay for my holidays or sick days.
If a family doesn't bring their child (because of sickness or their own holidays), they still pay the regular monthly rate. If someone was to take an extended leave, then I would work out some sort of deal. I have a family that just left for the summer; dad is a teacher. I will not charge them for their spot for a couple reasons. My own kids are now off school, I am taking the month of September off to have a baby and then they will come back in October, and they are a great family to deal with. Then again, they are part-time and it may be different if it was a full-timer. Part timers are just extra money for me!