Create your birthday calendar

Go to Gmail and click on Calendar in the upper left (or just go to http://calendar.google.com)
On the welcome page, add your info. Continue

Your 1st (default) calendar is made for you automatically. I like to have a separate calendar for birthdays and anniversaries vs. my personal stuff. This way, I can share the birthday calendar without having to share my every little personal reminder. If you want to do likewise, follow these bulletted steps. If you don't care whether all of your calendar items are on the same calendar, skip this part.
To create a 2nd calendar:

  • Click on the Settings link in the upper right
  • Click on Calendars
  • Click Create New Calendar button
  • Fill in the form. If you want the calendar to be viewable by others, be sure to check the box that says Make this calendar public

If you didn't create a new calendar and you want your default calendar to be viewable by others: go to the Settings link in the far upper right, go to Calendars, and click the link under Sharing.

Navigation:
There's a small month-view on the left. The arrows on either side of the month's name will shift the calendar by 1 month. You can click on a day, a week, or click and drag to highlight an area to see. You can also adjust your viewing selection with the tabs in the upper right of the page (pictured here).

Save a birthday:
Go to the appropriate day. Click inside the full-day area.
The left picture shows the full-day area when you can see the hourly schedule. The right picture shows the full-day area when you can see the daily view (ie: the whole area is a full-day area).

Type the event's title. If you have a special calendar for birthdays, make sure you select it in the dropdown menu. Click edit event details
Details:
When: click all day
Repeats: Yearly

Optional email reminders

On the right is an Options panel where you can set a reminder for this event. Email is an option on the Reminders drop-down menu

When you're done, click Save. I'm not sure why, but the Save button is near the top of the page. I included this picture because I don't think the button is easy to find.