Facebook used to display your birthday right on your Timeline, but at the time of publication, your birthday is hidden away in the About section of your profile. If people can't see your birthday, you may have never entered it or used a privacy setting that limits who can see it. You'll find the place to edit your birthday and designate who can see it in the About section of your profile.
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Birthday and Audience
Visit your Facebook Timeline and click the "About" link. Click "Edit" next to Birth Date or Birth Year under the Basic Information heading. Use the drop-down menus to change your birth month, date and year. The two privacy selector drop-downs next to the birthday fields control who can see your birth date and birth year. Set each to "Public" or "Friends" so others can see your birth date. Choose "Custom" to manually determine who among your friends can see your birthday. Alternately, use a friend list you've created to customize birthday visibility without manually entering each friend's name.
Facebook Help warns that users must enter their correct birthdate. While the site doesn't offer a specific number, Facebook also warns that you can only change your birthday a limited number of times, and you might have to wait a few days to change it again after first changing it.