The Calendar takes every birth and death in the file that has a full date and puts it on the day it happened, regardless of year. Open March and see everyone in the family born on each day of March, and every death anniversary, across the whole span of the tree. Every other dated event (a marriage, a burial, a census, anything the file records) appears in the year it happened, on its date. A date written as a day and month with no year at all, the way a slava, a name day or a yahrzeit is often recorded, is treated as yearly: it appears on that day every year, marked "yearly", labeled with whatever the file calls the event.
Two things people do with it
On this day. Look up today's date and see who in the family shares it.
Reunions and newsletters. A quick way to pull out the birthdays and anniversaries in a given month.
Putting it on your own calendar
The Calendar exports ICS files that any calendar app can import: birthdays of living people, birthdays of deceased people, death anniversaries, and a fourth file with everything else (marriages, burials, custom events and the yearly ones). One-off events land on their real date; yearly events are exported once, for the year you have open. Import the living-birthdays file into your phone and you will never miss a cousin's birthday again.
What is not there
Events with only a year, or a month and year, cannot go on a specific day, so they are left out; an approximate date that still names a day ("about 3 Sep 1820") is placed on that day. The Events page still shows the rest.
Next
Open the Calendar to your own birthday.