If you are using your desktop to add the event to your calendar some options make take longer to load.
If using Outlook on your desktop, please make sure you choose Outlook Desktop as your calendar option. An .ics file will download, and your downloads folder will pop-up on the right-hand side of your screen. Click Open File for the .ics file to finish adding to your calendar. If the downloads pop-up window doesn’t load, click the Add Again link in the middle of your screen.