Setting your birthday to "Friends of Friends" means that when your friends and their friends visit the About page on your profile, they will see the month and day of your birthday. In addition, the fact that your friends can see your birthday means that Facebook will notify all of your friends about your birthday. You don't need to change anything - your account is already set up for your friends to receive the birthday notification. If you don't want birthday notifications to go out to your friends, then you would have to set your birthday to "Only Me".
Setting your birth year to "Only Me" will not affect the birthday notification that goes out to your friends. It will only hide your birth year when people visit the About page on your profile.