You can either show your date of birth on Facebook or hide them. There are options where you can show just day and month not the year. To show/hide or just to show the date and month, go to Account -> Edit profile. From the dropdown box shown in the screenshot it has 3 options. 1. Show full date of birth, 2. Show only date and month and 3. Don’t show my birthday. When you set either option 1 or option 2 alerts will be sent to your friends on Facebook.