I don't think there is a way to do that monthly from within Nagios. You would need to write a script to scrape that info and send it out.
However, from within Nagios you certainly could set up weekly notifications. Just set a Timeperiod that is something like:
alias Once a Week Notifications
Next set up a special contact group:
alias Weekly Notify
Notice I chose only to be notified of warnings.
Finally add the contact group to the services you wish to monitor
use generic-service ; Name of service template to use
contact_groups admins, Week
Notice I added the contact group week.
You might have to mess with the times so that you get a notification only once, but that would depend on how you have notifications set up now.
Anyway, I think this gives you what you want on a weekly basis. If you really wanted to be super fancy I guess you could create a script to restart nagios using different config files for one day a month or something.
Hope this helps.