If you're on Python 2.4, then consider whether or not you can use a TimedRotatingLogFileHandler from the logging module to handle this for you: http://www.python.org/doc/2.4.1/lib/node344.html
Of course, that only works if defining a "month" as 30 days is acceptable. If you genuinely need calendar months, then you still need to do it manually. -- http://mail.python.org/mailman/listinfo/python-list
