Can I backup my calendar from my phone to i cloud

There are two options. You can back it up as part of the overall iPhone backup, or you can sync it with iCloud. A backup can only be used to restore the entire phone, not just the calendar. But a sync is real time and the calendar can be viewed on other devices that share the same iCloud ID, as well as at https://icloud.com.

To sync go to Settings/<your name>/iCloud and turn on Calendar. If you just want a backup go to Settings/<your name>/Backup.