There's a bit of a trick to scheduling a Master Aero for the last day of every month, but it's not hard.
- Create your Master Aero as usual.
- When you get to the Occurrence Options part, choose the 31st of the month as your Start Date. If the month you want the Master Aero to start on doesn't have 31 days, go back a month and select the 31st of that month.
- Check the box for Day of the Month
- Click the Insert Occurrences button.
- The Master Aero will be scheduled for the last day of every month.
- If you have an extra occurrence in the beginning that you don't want, click the View button on that occurrence and delete it.
- Save & Close the Master Aero.