You told the terms that you needed when you were hired. That gave them clear and advance notice that events on Saturdays during those times would require a substitute musician. It is hard to see how that could be any clearer. The ball is in their court, but they have NO reason to be angry or accusatory toward you. If the terms are no longer acceptable to them, they could give you notice(then the ball is in your court) and hire another musician who will work at all times. If you are a steady as you have indicated, you wouldn't have any problem finding other employment. This is simply business and should be done without rancor. Churches have spiritual/business functions and they are separate but necessary. Sometimes when working on a business matter, officials try to make it spiritual and get all emotional and unreasonable about it.
If you have a musician who is working on a spiritual basis and would play at any need for low or no pay, it is still unreasonable to accuse them of being less than spiritually diligent because they sometimes have things that they must attend to even when they are needed. They are allowed to have a life and still be spiritual. So do you.
Of course, if enough money is being paid, and no stipulations/special allowances have been made and agreed to, one might be expected to appear at all events except in cases of dire emergencies. No accusations or guilt trips: you come or you're gone. Just business.
