Adding an ICS calendar to your iOS device -- such as an iPad or iPhone -- is not done through the Calendar app. The settings to subscribe to ICS calendars are actually located in the Mail, Contacts, Calendars section of your device's Settings menu. You need the ICS calendar's full URL -- which is an ICS file -- to add it to the Calendar app. After you subscribe to the calendar, it appears under "Subscribed" in the Calendars section of the Calendar app.
Tap the "Settings" app on your iOS device's Home screen.
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Select "Mail, Contacts, Calendars," then tap "Add Account" in the Accounts section.
Tap "Other," then tap "Add Subscribed Calendar."
Enter the ICS calendar's address in the "Server" field.
Tap "Next," then enter a name for the calendar in the Description field.
Tap "Save" to add the calendar to the Calendar app.