The birthdays calendar
In addition to the calendars you create, you'll also see a special calendar called Birthdays. You can turn this on and off in the calendar view menu in the toolbar.
The Birthdays calendar is created automatically from your contacts. It will show the birthdays of every person who has a day and a month of birth in their individual contact card. For contacts with a specified year of birth, the events will say how old they will be turning on each birthday.