I don't think you can do this using CLI or System Manager. I think Protection Manager can do it, but unless you're already using Protection Manager, I don't think it would be worth the effort of setting up Protection Manager just to schedule monthly snapshots. If I were tasked to do this, I would probably look into using a Windows or Unix host to "schedule" a monthly task/script to take the snapshots and manage the retention of the snapshots. We already have a "SAN Manager" windows server that does a bunch of these "administrative" tasks like schedule daily perfstats, act as our DFM server, etc.
Keeping 26 or 30 weekly snapshots would have a similar effect. Other than that I would use a Unix shell script using SSH to do it. If you're not Unix you should be able to use PowerShell or SnapDrive for Windows (if a Windows lun) and the task scheduler.
What is the platform and protocol you are using?