For most monthly events it is best to use the "n* per week" choices.  Using the weekly option you can specify events that occur on the 1st Tuesday of the month or the 4th Wednesday etc,  

The "month" option is suitable for specifying a day of the month.  In this example we specify and event that occurs on the 12th of each month.

The "Month" option can cause confusion - consider the following example 

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This event will occur on the 1st Monday on or following the 15th of each month.