(1) Most twins share the same birthday (Day, Month, Year).
(a) An exception occurs when one twin is born before midnight and the other is born after.
(b) Another exception occurs when one twin is born on New Year’s Eve and the other is born on New Year.
(2) If there are more than 13 people, then at least two must share the Month in their birthdays. It follows the Pigeon Hole Principle.
(3) If there are more than 30 people, then there is a high likelihood that at least two share the Day and the Month in their birthdays. It follows from Probability and Statistics.
(4) In a group, the birthdays are not equally distributed. For instance, my two sisters and I are all born in August.
(5) Some restaurants do not charge the Birthday Person (Boy, Girl) is there are three more persons in the group. They will ask for the ID.
(6) “Happy Birthday” song is one of the most played songs.