If you already have birthdays recorded in your calendar, you can connect one or more of them to a contact using the Item command on the Insert menu.
In the Insert Item dialog box, you would select the Calendar folder, because you want to connect calendar items to the contact. Notice how the calendar events in the Items section of the dialog box are grouped by recurrence.
You would select from the Recurrence : Yearly group because . . . birthdays and anniversaries are events that occur every year.
Note You can connect any calendar entry to a contact. But to keep it simple, we'll focus on birthdays.
After clicking the calendar and selecting a calendar event to link to the contact, you would select an insertion under Insert as type. In this case, you'd select Shortcut. A shortcut allows you to jump directly from the contact to the event.
Note You don't want to select Attachment—this insertion type would create a complete copy of the event inside the contact, much like attaching a document to an e-mail message. This copy would actually not be stored in the calendar.