Add a birthday to someone’s Address Book Contact?

First off, I’m amazed at how many different names there now are for Mac applications. What used to be known as Address Book is now Contacts. What used to be known as iCal is now simply Calendar. Or do we call it “Mac Calendar” or “Apple Calendar”, to be correct? In any case, I’m going to assume that you are running the latest version of Mac OS X so that you have the latest apps, but actually the way you add a birthday to someone’s contact record is the same as it’s been for a long time.
It’s just hidden.
Well, maybe “hidden” is a bit dramatic. It’s accessible, but you have to know to look on a menu rather than have it show up as a field on what turns out to be the “template” for an address record.
But let’s stay focused. Here’s a new record I’m adding for my generically named friend John:

As you can see, there are fields ready to be filled out for their mobile number, home email, home page URL, address, etc. But where’s the “Birthday” field?
That’s one of the fields you can find from “Add Field” on the Card menu:

Look at all those options! You can add their maiden name if they’ve just gotten married, a date (though I’m not sure what that’s for), IM or Twitter handle, profile, etc. Lots of stuff. You can also go and tweak your “template” to change the default fields if you really like tracking birthdays or never enter their AIM handle.
Choose “Birthday” and a new field pops up:

What’s initially confusing about this is that there’s no clue about what format Contacts wants you to use when entering the date. Turns out you can use a bunch of different formats, it’s pretty smart (seems to be the same date parser as in Calendar, to no surprise):

Notice that I’ve abbreviated the month name and the year, but when I press Tab it’s expanded to the correct, formal date:

Now here’s a neat trick. Having saved this record (click “Done” on the card) quit and restart Calendar, then go to the date of their birthday:

Not only does it show up as a handy reminder, but it also calculates how old they’d be, so as you can see my pal John just turned 26. Nice and easy, and if you can get your friends to share their birthdays, you’ll have a great list all neatly in your Calendar (and therefore also on your iOS devices if you sync or use iCloud).