iCloud: Show the Birthdays calendar
In iCloud Calendar, click the Action pop-up menu below the sidebar, then choose Preferences.
In the General pane, select “Show Birthdays calendar,” then click Save.
Show or hide the Birthdays calendar by turning it on or off in the Subscriptions section of the Calendars sidebar.
To modify a birth date, use iCloud Contacts; changes are reflected in iCloud Calendar within a day.
You can also see the Birthdays calendar on your iOS devices and Mac computers by selecting it in the Calendar app.