You can backup at anytime. It's the restore that will take your system out of service.

There is a setting for Automatic Backup, can be set Daily or Weekly. Default is Sunday at 2am.

Be sure it is on. Inherited one customer, didn't have automatic backup turned on, and the Receptionist went to renumber an extension, got to dial plan instead, and defaulted all the extension numbers and dial codes. Quite the mess to sort out!