You can look at Omnifocus (a task management app), which has a repeat task option.
They split it into two dropdowns once you select Repeat monthly.
Option 1: select specific days for the month:
Option 2: select days of the week for each month
##Update: Another option - make it read like a sentence
Update: Another option - make it read like a sentence
You could also make the form to read like a sentence.