why aren't you keeping her days scheduled? does she show up just any random two days she wants? could you leave for the day tomorrow? take a little outing somewhere, during her time of drop off? then when she calls to ask why you weren't there for her child? you can explain you were under the understanding that she was keeping the same days as last month, given she didn't inform you otherwise, so you planned an outing the next day not thinking her daughter would be in attendance.