For email centre->Delivery options->Sending schedule: if you select the Repeated option, there is no form validation or indication of required fields.
If you select Every month, but don't supply an 'Every' value, you get a 500 error. It's also not indicated if the Scheduled start/end dates are mandatory, you can submit this blank with no error
Nice one, thanks for the report. We will work these into the 10.10 release with other Email Center enhancements