You share your birthday with approximately 18 million people worldwide.

If we assume an equal probability of being born any day of the year (not counting February 29th), every person shares their birth date with about 1/365 of any given population.

Therefore, you sing 'Happy Birthday' on the same day as approximately 824,456 Americans, 3 million Indians, and 3.6 million Chinese. However, you would only be singing with 2 or 3 residents of Vatican City!