You would like to know when donors are charged for their recurring monthly donations.
Monthly donations are charged every 30 days from the day that the payment was set up. So the day is not always the same each month, as the subsequent donation date is based on the last donation date plus 30 days.
Administrators can change the Next Donation Date from People -> Donors by clicking on the donor in question. There, update the Next Donation Date as needed. Donors cannot customize their next donation date.