Why does the “Add to Calendar” link only add the first day of the event to my Outlook calendar?

This is most likely due to a “quirk” specific to Outlook regarding how you import .ICS files into Outlook. If you click on the .ICS file link and choose “Open” or “Open with Outlook” it will result in Outlook only importing the first day of the event into your calendar. You need to save the .ICS file to your computer and then use the “Import .ics or .vcs file” process described on this Microsoft support page. Follow those Microsoft instructions up until the last step, instead of selecting “Open as New,” select “Import” to properly import all the event days into your Outlook calendar.