.ICALENDAR File Extension
What is an ICALENDAR file?
Universal calendar format used by calendar programs such as Google Calendar, Apple Calendar, and Microsoft Outlook; saved in a plain text format and contains an event or task; used for sending events or tasks to others, who can import it into their calendars.
The ".icalendar" extension is not as commonly used as the .ICS extension.
NOTE: Apple Calendar was formerly known as Apple iCal but was changed in 2012 with the release of OS X Mountain Lion.