Schedule iCloud backup

iCloud backups can be set to automatic, but they can't be scheduled.

Note that for a backup to occur all of several conditions must be met. A schedule can happen only when the conditions are the same always, and this cannot be assured for a portable device.

See How to back up your iPhone or iPad with iCloud – Apple Support (UK)

Back up automatically with iCloud Backup

1. Make sure iCloud Backup is turned on in Settings > [your name] > iCloud > iCloud Backup.
2. Connect your device to a power source.
3. Connect your device to a Wi-Fi network.
4. Make sure your device's screen is locked.
5. If you receive an alert that states you don't have enough iCloud storage space to complete the backup, follow the onscreen steps to buy more storage.