A lot of people who use Facebook have their birthdays and anniversaries listed on their profile. Facebook has a built-in calendar that helps you keep track of all your friends' and family's birthdays and anniversaries.
Since Facebook is integrated with iOS and OS X, you can also add Facebook events to the Calendar app on your Mac. This makes tracking events easier as you don't have to login to Facebook just to check out upcoming events.
It is relatively easy to add/sync Facebook events to your Calendar on Mac. There are, incidentally, two ways to do this.
Method #1: Add/Sync Facebook Events to Calendar on Mac Via System Preferences
Step #1: Click on the Apple Menu and then on System Preferences.
Step #2: Choose Internet Accounts.
Step #3: If you haven't added your Facebook account already, click on + in the left side-panel.
Step #4: Add your Facebook account details to add the account to this list.
Step #5: Now, in the right pane, tick the Calendars option (to enable it).
Step #6: Quit the Calendar app and restart it. After the sync is complete, Facebook events should show up in your Calendar app.
Method #2: Add Facebook Events to Calendar on Mac Via the Calendar App
Step #1: Open the Calendar app.
Step #2: Click on Calendar from the menu bar.
Step #3: Now click Add Account.
Step #4: Select Facebook option from the dropdown. Fill in your Facebook credentials.
Once you are done adding the Facebook account, all events from this account will be synced to your Calendar. Depending on your network and the number of events, it might take a little while before all events are synced.
Both these methods work on OS X Mavericks and later. Older Macs did not feature Facebook integration of this sort so you won't be able to sync Facebook events to the stock Calendar app on Macs running older versions of OS X.