Go to OSX Daily to read How to Change Calendar Event to iCloud on iPhone & iPad
Sometimes when adding an event to Calendar on iPhone or iPad, you may unintentionally add the event to the calendar on your local device, rather than to iCloud. This is often why a calendar event appears to not sync with other Apple devices, because if the event is stored in a calendar set to “On My iPhone” or “On My iPad” rather than iCloud, it will not sync between your devices.
Similarly, if a calendar event is set on a local calendar, you will not see the option to send an invite to that calendar event as the ‘Invitees’ feature is unavailable for…
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