The calendar is a listing of events related to your family that you may be interested in. It currently only displays birthdays and anniversaries (and corresponding ages), but will soon also includes upcoming family events you may wish to attend, death anniversaries (in certain cultures, these days are used as mourning and remembrance days for the deceased person; off by default).
The calendar only displays calendar events for members of your Family group with a Date of Birth or Marriage entered. If a birthday or anniversary is missing from your calendar you can add it by editing that person's profile.
Note: Make sure when entering dates in a profile that they are input correctly and with a complete 4-digit year. An improperly input date may not display on the calendar.