Enable the Birthdays calendar

If you include people’s birth dates in the information you have in iCloud Contacts, you can enable a special Birthdays calendar that displays them all.

Show or hide the Birthdays calendar

In iCloud Calendar, choose Preferences from the Action pop-up menu below the sidebar.
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 birthdate, 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 (or iCal in OS X v10.7.5).