ICS
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ICS (AKA iCal or iCalendar file / feed) is a format for calendar files (a form of feed file) used by many programs such as Apple Calendar and Google Calendar, and can be used to share and subscribe to calendars across different systems.
Why
You may want to publish a private (by obscurity) ICS feed on your site in order to subscribe to it from:
Both of which are useful calendar readers
How
Publishing an ICS feed is a bit of non-trivial quirky work, as every "consuming application" (Calendar app) supports slightly different versions / overlaps of ICS.
You can either write code to generate your ICS (and cache it statically or generate it on request), or use a service like H2VX to convert a page of hCalendar (or h-events) into an ICS.
How to validate
IndieWeb Examples
Aaron Parecki
Aaron Parecki implements an ICS feed on his site aaronparecki.com since 2016-03-24.
Marty McGuire
Marty McGuire implements an ICS file for each event post on his site since 2017-07-13.
gRegor Morrill
gRegor Morrill implements an ICS file linked at the bottom of each event on his site since 2017-07-27. Uses mf2 to iCalendar to convert h-event markup to ICS.