There is some information that I am good at remembering (network IP addresses, for example), but other information that I'm not so good at remembering. Birthdays, unfortunately, falls into the category of things that I have a hard time remembering, and I'm not sure why (put it down to my being human). Fortunately, the Calendar app (in conjunction with the Contacts app) in OS X helps me with this so it looks like I remember everyone's birthday.
In order for Calendar to be a faithful reminder tool, we must first do a couple things to help it out:
Enter Birthday Information
The first thing we need to do is to actually enter the birthday information for an individual in the Contacts app (you'll see why in a minute). To do this, open Contacts, locate the individual's contact entry (or create a new one), and enter the birthday information (matching the suggested format), as shown in the example below:
View Birthdays in Calendar
Now that we have birthday information in the Contacts app, we need to make sure that this information will be displayed in the Calendar app. To do this, open the Calendar app, then click on the "Calendar" button in the top left-hand corner (to reveal your list of calendars). Make sure a check is placed in the box next to "Birthdays" list.
By turning this feature on, it will actually refer to the Contacts app for the birthday information (that's why the first step was necessary), and display it within the Calendar app. As you can see, the more birthday information you have in your Contacts app, the more useful it will be in remembering birthdays:
Turn on Birthday Alerts
Now for the magic sauce... automatic reminders. By default, Calendar will remind you of birthdays in your calendar on the day of the birthday (specifically, at 9:00a). In most cases, this may work, but if you like a bit more notice before someone's birthday (in order to send a card or gift), you will want to be reminded about a week in advance. Calendar makes this simple (and automatic) as well.
To set a reminder one week ahead of each birthday, go to Calendar > Preferences (or Command + ,). On the "Alerts" tab, change the birthday reminder time from "On Day of Event (9 AM)" to "1 Week Before".
With all of these settings in place, you will not only see birthdays in your Calendar (which will also sync to your other iOS and Mac OS X devices), but you will also be reminded of the birthdays one week before they happen. You will look like a genius as you remember all these birthdays!