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.
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
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.
How to validate
Aaron Parecki implements an ICS feed on his site aaronparecki.com since 2016-03-24.