Asalaamu Alaikum, there is a difference of opinion regarding the exact birth date of Rasulallah (Sallallahu Alaihi Wasallam). Some scholars say he was born on the 12th but the vast majority of scholars state that he was born on the the 9th of Rabiul Awwal. The fact that his exact birth date is disputed proves that knowing the exact date of his birth is not a part of religion as his birth was never commemorated by any of the Sahaba or earl salaf.