While I don't think this is acceptable to be getting notably upset about I can understand her confusion. If you check the Calendar she is starting the first week of December, it just happens that December 1st is a Tuesday. So, depending on how you look at it and how her paydays work it could be considered paying for an extra day if she is used to biweekly payments. But again, I find it concerning that she is still notably upset about this after you have explained it to her.

I would also suggest switching to biweekly payments or paying by the number of worked days in a month because charging a flat monthly fee means you work extra days for free many months of the year.