Announcements tab in Administration -> People help you set up and use different popups showing up when users log in. Birthday announcements are default reminders and you cannot modify them. They are active if you enable the announcements settings (Use announcements and Show birthday announcements) from the General tab.
In order to add a new announcement, click on New announcement and fill in the fields. It can pop up every day, every week, every month, or even only once. In order to make it pop up, you have to make it active. Also, there are different types of announcements - warning, error, or notice. They differ in color.