Subscribing adds the organizer's calendar to the Calendar app on your Mac. New and changed events then appear automatically.
Steps
Open the calendar page you received from the organizer and click Add to Calendar. If the page asks for your name and email first, fill them in and continue.
Choose Apple. Your browser asks whether it may open the Calendar app. Click Allow (Safari) or Open Calendar (Chrome, Edge, Firefox).
Calendar opens with the link already filled in. Click Subscribe.
Check the settings for the calendar and click OK:
Location: choose iCloud to see the calendar on your iPhone and iPad as well, or On My Mac to keep it on this Mac only.
Auto-refresh: this is often set to Every week. Change it to Every hour or shorter, otherwise new and changed events can take days to appear.
The calendar now appears in the sidebar of Calendar.
Good to know
Updates follow Auto-refresh. New or changed events appear the next time Calendar refreshes, so with Every week that can take up to seven days. To refresh now, choose View > Refresh Calendars.
Read-only. You cannot edit events in a subscribed calendar.
Unsubscribe
Right-click the calendar in the sidebar of Calendar and choose Unsubscribe.
Troubleshooting
Nothing happens when I choose Apple. The browser prompt was dismissed or blocked. Choose Apple again and allow the browser to open Calendar.
Another app opens instead of Calendar. A different app is set as your default calendar app. Change it in Calendar > Settings > General > Default calendar app.