Create a calculated field called [Last] with the following formula:
LAST() = 0
This is a table calculation that will return True only for the last row of each partition (in your case, you want True for the last Month).
Add that to filters, set to "True" & set addressing of the table calculation to Month (everything else should be in Partition).
In you can't get it to work, post a packaged workbook.
Thanks Steve. It worked like a charm.