Hello! If you delete a calendar that you have synced or backed up with iCloud, you can restore it via iCloud.com. I would recommend that you do this from computer. You navigate to iCloud.com and sign in with your Apple ID information. Once you sign in you will click the settings icon. When this loads you can scroll to bottom of the page and there will be an option to restore calendars and reminders. When you click that link it will show all available archive that you can restore.
If you do not see any archives it may mean that the calendars you were using were not iCloud/Apple calendars or you may not have been backing up your device to iCloud.
Let me know if this helps!