When you switch on the calendar, a calendar option appears on the top menu of user-side pages. Members (not guests) have the option of subscribing to the calendar for notification of new events. Your usergroup permissions (Admin -> Members -> Manage Usergroups) determine who can submit and edit events.

At Admin Panel -> Settings -> General you can set how recently the member needs to have logged in for their birthday to be listed. If you have a very large number of members (10,000+) you may notice a slowdown in viewing the calendar, and can speed things up by setting it to show fewer birthdays.