How to add birthdays and anniversary to a contact

In Contacts, click the person you want, click "Edit," scroll down to the bottom and select "add field," and then pick "Birthday" from that list. You can also choose the "Date" field to add another date (such as an Anniversary).

I'm not sure of another way that's not "manual." You might be able to do something like... if you are linking your contacts to your gmail contacts, export your list, add birthdays to everyone, reimport it.... Or maybe someone who's more familiar with using Address Book on a Mac might have a different idea. I mostly manage mine from gmail, and from everything I've seen (minus export/import option), it's a one-by-one process.

When editing a Contact, click the Plus sign next to "add field."

Scroll down through the list, and Birthday is an option you can add.