For our requirement, we were taking two level backups = Daily backups and Monthly backups. The daily backups are recycled every week and monthly backups are stored for much longer duration [As a matter of fact, we have not yet deleted them for past 3 years]. Of course, our backups are only 200+ MB now.

So we have ended up with:
01-2008.dmp, 02-2008.dmp, 03-2008.dmp, … 05-2009, 06-2009.dmp, etc and seven daily dump files = Mon.dmp, Tue.dmp, Wed.dmp, … Sat.dmp, Sun.dmp

We used `date +%e` to get the day of today. We used `date +%0m` to get month and `date +%0y` to get year.