Hi! I'm not a teacher.
Per means 'each'.
In this context, it refers to each last day of a calendar month. For example, the last day of each month varies from 30th to 31th, and the last of Feb. is 28th.
Cambridge Dictionaries Online - Cambridge University Pressper
used when expressing rates, prices or measurements to mean for each:
The meal will cost $20 per person.
The car was travelling at 70 miles per hour (70 mph).
There are more cafés per square mile here than anywhere else in the country.
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