When creating an recurrent appointment on the 31st of the month Outlook gives the user a warning: "Some months have fewer than 31 days. For these months, the occurrence will fall on the last day of the month."  And as the message indicates, the event occurrences would start on the last day of the month when the 31st does not exist. 

This will set the occurrences at the end of each month regardless of the length of the month.