I charge by the month. I find that this works better for book keeping. Every month they pay the same amount. I charge as if there are only 4 weeks in a month and I bank the days I have not been paid for as holidays, with parents still paying the same amount. You could charge this way or charge for 4.5 weeks per month ---eg--$40 per day X 5 days per week = $200. X 4.5=$900.per month. I put a note on my door reminding them of the date that payment is due.
I prefer parents paying at the beginning of the month as I have had some parents with late payments. I can't imagine these parents being on time for payment every week.
What ever works for you stick to your guns and make sure that you get your payment on the designated day.