There’s an open standard for calendars called the iCalendar specification that lets programs like the Calendar app on macOS subscribe to online calendars and auto-update. Most calendar apps will support this format, including Google Calendar.
How to Add a New Calendar Subscription
Open the Calendar app from the Dock or your Applications folder. From the main window, you can add a new calendar subscription by choosing File > New Calendar Subscription from the top menu bar, or by pressing Option+Command+S.
This will open up a window asking you for a URL, which should be a link to a file ending with an .ics extension. Depending on what calendar you’re trying to subscribe to, the method of getting this link may vary. We’ll be using Google Calendar in this example.
For Google Calendar, you can find the link by opening Settings, clicking on the calendar in the sidebar, and scrolling to the very bottom to find a box labeled “Secret address in iCal format.”
Copy this address and paste it into the Calendar app’s dialog. You’ll get an error message if the URL wasn’t an iCalendar link, but if it worked, you’ll see this screen with the subscription settings.
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Source : How to Subscribe to Calendars on Mac
