Events+ recurring events every month option

I've tried everything I can think of to get a recurring event set up that is once per month on a specific day of the month. I've set the event up, no problem. Made it recurring, no problem. The problem is that when I select the repeat every month option, there is a blank field next to "On". I've tried "Tuesday", "1st Tuesday", etc, but none of these work, and the event will not be registered on the calendar or events list. What is the proper format for the "On" field?