Everyone who creates a Facebook profile is required to give a full and accurate birth date to the site, but you can hide the date for privacy purposes. If you have previously hidden your birthday but want to share it, editing your profile takes just a few seconds and the changes occur immediately. When your birthday is exposed, friends receive a reminder about your birthday beginning the day before.
Click on the "Profile" link at the top of any Facebook screen to go to your personal profile.
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Click the gray "Edit Profile" icon in the top right corner of your profile.
Go to the "Basic Information" link, located on the left side of the screen, and scroll down to the "Birthday" section.
Click on the drop-down menu located just beneath your birthday and select either "Show Only Month & Day in My Profile" or "Show My Full Birthday in My Profile."
Scroll down to the bottom of the page and click the blue "Save Changes" button.