Who do you know with the most popular birthday? The research varies. Anybirthday.com crunched data from millions of records and they claim October 5th is the most common day to be born. Why? If you do the math and count backward, conception could have fallen on New Years Eve. Happy Birthday to mirror twins, Betsy Belz and Sally Graflund of Memphis, share the birthday Oct. 5th and the pair turned 70 today.