I'm building an open-source Calendar as my senior project, I'm able to get my .ics files to validate on iCalendar.org and work on Google Calendar, but Apple's calendar doesn't accept it.
Apple released their own calendar server under the Apache 2.0 license.<p><a href="https://www.calendarserver.org" rel="nofollow">https://www.calendarserver.org</a>
Quick Google, this might help. Best of luck on the project. <a href="https://developer.apple.com/documentation/devicemanagement/caldav" rel="nofollow">https://developer.apple.com/documentation/devicemanagement/c...</a>