If they are consistently needing those days than I would be charging them for 3 days each week at my daily part time rate regardless if they do not 'need' the care on the alternating Friday they are wanting it saved to be there for them then they pay for it ... my policy is payment in advance of care either weekly, bi weekly or monthly whatever works best for their budgeting and this is agreed upon at the signing of the contract and than fees are due on the Friday by noon for the upcoming week(s) of care on whatever schedule they have chosen.

I would than advertize to fill the Tues / Thursday space they have vacated.