If you go to Manage -> Schedule Menu you can create your own custom schedules as shown below:
Because February is a 28 day month you would need to create a single schedule for February with the day and weekday you intend to use. In the example below it is the last Saturday of the month.
Then create a second schedule that encompasses all of the other months (exclude February) and put in the days 24-31 if your intent is the week of the month.
The system will honor both the Days of the Week and the Days of the Month and both conditions would have to be met to show up on the Audit/Dashboard screen. So in this example we would only expect and show this backup on the last Saturday of the month.