Traditionally 12th Rabee u-Awwal is considered to be the Day of the Birth of the Messenger of Allaah (Sallalahu Alayhee Wassallam) and this is the day the soofee and bareilwees gather to indulge in this innovation.
However from a historical perspective this seems to be an incorrect day and the scholars of Islaam have differed as to the day the Messenger of Allaah (Sallalahu Alayhee Wasallam) was born.
The Seerah of Ibn Ishaaq mentions the Messenger of Allaah (Sallalahu Alayhee Wassalam) was born in the year of the elephant ie when the army of Abraha attacked the Kab’ah which was compromised of elephants thereby incurring Allaahs anger. (with a good chain from Ibn Ishaaq as was said by Shaikh Abdur Rahmaan al-Banna in his Fath ar-Rabbaanee (20/190)
Imaam Suhailee mentions, “Elephants came to Makkah in the month of Muharram and the Messenger of Allaah (Sallalahu Alayhee Wasallam) was born 50 days after this event.” This also seems to be the opinion of the majority of the scholars according to Imaam Suhailee and Muhammad ibn Ishaaq. (Fath ar-Rabbaanee (20/190)
The famous historian Haafidh Ibn Katheer writes in his well known book of history, al-Bidaayah Wan-Nihaayah that the majority of the people of knowledge believe the Messenger of Allaah (Sallalahu Alayhee Wassalam) was born in the month of Rabee u-Awwal however there are varying statements pertaining to the exact date, whether this was in the beginning of the month, at the end or during the middle. Some have opined 2nd of Rabee ul-Awwal, some have said 8th Rabee ul-Awwal, some have said 10th, some have said the 17th, some have said the 18th, and some have even said the 21st of Rabee ul-Awwal. The dates that have precedence are the 8th of Rabee ul-Awwal and the 12thof Rabee ul-Awwal. Haafidh Ibn Katheer himself has given precedence to the 8th of Rabee ul-Awwal. This is also what Imaam Humaidee has transmitted from Imaam Ibn Hazm and many other scholars have also supported this date. (al-Bidaayah Wan-Nihaayah (2/259-262) of Ibn Katheer)
Imaam Tabaree and Ibn Khaldoon have favoured 12th of Rabee ul-Awwal (cited from Allaamah Qadhee Suleimaan Mansoorpoorees book, Rehmatul Lil-A’alameen (1/40)
Imaam Ibn al-Jawzee has favoured the opinion that the Messenger of Allaah (Sallalahu Alayhee Wassalams) date of birth was on the 10th of Rabee ul-Awwal in his book ‘al-wafaa Bi-Ahwaal al-Mustafaa’ (1/154).
In recent years 2 well known biographers ie Allaamah Qadhee Suleimaan Mansoorpooree in his Rehmatul Lil-A’alameen and Allaamah Shiblee in his Seeratun-Nabee, both opined the date of the birth of the Messenger of Allaah (Sallalahu Alayhee Wassalam) was on the 9th of Rabee ul-Awwal corresponding to 20th April 571 ce (Shiblees Seeratun-Nabee (1/171), Allaamah Qaadhee Suleimaan Mansoorpoorees Rehmatul Lil-A’alameen (1/40).
The same date has been endorsed to be correct by Muhammad Tala’at in ‘Taareekh Duwal al-Arab’ (cited from Qaadhee Suleimaan Mansoorpoorees Rehmatul Lil-A’alameen (1/40) footnotes and (2/367) and also refer to Muhammad al-Qudwah al-Kaamilah (pg.7)
(From ‘Celebrating Milaad On the Day Of Messenger of Allaah (Sallalahu Alayhee Wasallam) Demise’ of Shaikh Muhammad Munir Qammar with slight adaptation (pgs. 11-12)