This is how to subscribe to an internet calendar via a link (usually ending in .ics) in Outlook, e.g. public holidays or appointments from another system.
Outlook (classic)
Go to Calendar.
Choose Home › Add Calendar › From Internet ….
Paste the link and confirm with OK.
Confirm the prompt with Yes.
New Outlook and Outlook on the web
Go to Calendar.
On the left, choose Add calendar › Subscribe from web.
Paste the link and enter a name.
Click Import.
Note: Subscribed calendars are read-only. Changes in the source appear with a delay, sometimes only after a few hours.