In the name of Allaah, indeed all praise is due to Allaah and may peace and blessings be upon the Messenger of Allaah.
The Prophet (sal Allaahu alayhi wa sallam) informed us that some of the Muslims would begin to follow the ways of the non-Muslims. He said:
Surely, you will follow the ways of those nations who were before you, so much so that even if they entered a hole of a lizard, you would enter it.
They said, “O Messenger of Allaah, do you mean to say that we will follow the Jews and the Christians?”
Whom else [other than them]? 
Out of their love for Jesus (Eesa), the Christians innovated into their religion by specifying the 25th of December as his birthday and celebrating it, even though there is no origin to it.
Unfortunately Muslims came later, and out of their love of the Prophet Muhammad (sal Allaahu alayhi wa sallam) they specified 12th Rabee’ al-Awwal as his birthday, which also does not have an origin. The Prophet (sal Allaahu alayhi wa sallam) as well as his companions never took this as a day of celebration.
This innovation, Mawlid an-Nabawi, is not only not legislated but even historically incorrect. It cannot be established for certain that the birthday of the Prophet was 12th Rabee’ al-Awwal. The scholars greatly differed over the actual date of the birth of the Prophet (sal Allaahu alayhi wa sallam). By the permission of Allaah, this article will outline some of these opinions and therefore establish the historic invalidity of specifying the 25th of Rabee’ al-Awwal as the date of his birth.
The difference of opinion with regards to the Mawlid 
1. 1st of Rabee’ al-Awwal: Mentioned by Muhammad ibn Muhammad ibn Munhad [in It’haaf al-Waraa].
2. 2nd of Rabee’ al-Awwal: Mentioned by Ibn Sa’d [in Tabaqaat], Ibn Katheer [in Bidaayah wa an-Nihaayah] and Zarqaanee [in Sharh al-Mawaahab].
3. 8th of Rabee’ al-Awwal: Mentioned by Ibn AbdilBarr [in al-Isti’aab], Ibn Hajr al-Haythami [in Sharh Matn al-Hamziyah] and al-Qastalaanee [in al-Mawaahab al-ladaniyyah].
4. 9th Rabee’ al-Awwal: Mentioned by Shiblee an-Nu’maani [in Seerah anNabi], Sayyid Sulayman al-Masufooree [in Rahmat al-‘aalameen] and also Safee ar-Rahmaan al-Mubaarakfooree in [Raheeq al-Makhtoom]
5. 10th Rabee’ al-Awwal: Mentioned by Al-Waaqidee [in Taareekh], Ibn Katheer [in al-Bidaayah wa an-Nihaayah] and Ibn Sa’d [in Tabaqaat]
6. 11th Rabee’ al-Awwal: Mentioned by Ibn al-Jawzee [in al-Muntadham]
7. 12th Rabee’ al-Awwal: Mentioned by Ibn Hishaam [in Seerah an-Nabawiyyah], Ibn Hibbaan [in ath-Thiqaat] and al-Bayhaqi [in Dalaail an-Nubuwwah]
8. 17th Rabee’ al-Awwal: Mentioned by Ibn Dahiyyah by some of the Shia scholars [in al-Bidaayah wa an-Nihaayah]
9. 18th Rabee’ al-Awwal: Mentioned by Ibn Katheer, he said this is the opinion of the Majority [in al-Bidaayah wa an-Nihaayah] and also Muhammad Husayn Haykal, he narrated it from Ibn Is.haaq [in Hayaat Muhammad]
10. The first Monday of Rabee’ al-Awwal: Mentioned by Ibn AbdulBarr [in al-Isti’aab] and Ibn Sayyid an-Naas [in ‘Uyoon al-Athar]
11. 18th Ramadhan: Mentioned by Ibn Kalby and also Zubayr ibn Bakkaar [narrated by Ibn Hajar in Fat.h al-Baaree]; also mentioned by Ibn Asaakir [Tareekh ad-Dimishq]
12. Before the Year of the Elephant by 15 years: Mentioned by Kalby, upon the authority of Abu Saalih, upon the authority of Ibn Abbaas
13. There are ten years between his birth and the Year of the Elephant: Mentioned by Ya’qoob al-‘Atamy, upon the authority of Ja’far ibn Abee al-Mugheerah…
14. The month of Muharram: Mentioned by Haykal [in Hayaat Muhammad]
15. The month of Safar: Mentioned by al-Qastalaanee [in al-mawaahib] and Muhammad Hussayn Haykal [in Hayaat Muhammad]
16. The month of Rajab: Mentioned by Muhammad Husyan Haykal [in Hayaat Muhammad]
17. Rabee’ al-Aakhar: Mentioned by al-Qastalaanee [in al-Mawaahib]
18. The day of the Elephant: Mentioned by Hajaaj ibn Muhammad upon the authority of Ibn Abbaas [in Seerah Nabawiyyah of adh-Dhahabee]
Eighteen different opinions have been mentioned above, some stronger than others, which demonstrates the historical inaccuracy in specifying the Mawlid an-Nabawi on the 12th Rabee’ al-Awwal.
The fact that so many different opinions exist, and yet no authentic Hadeeth, also shows that the Prophet (sal Allaahu alayhi wa sallam) himself, the noble companions (may Allaah be pleased with them), the great Imaams of Hadeeth and early pious scholars attached no importance to this matter. 
May peace and blessings be upon our beloved Prophet, his family, companions and the believers who follow his guidance until the Day of Resurrection.
Written by the one in need of Allaah:
Abul Abbaas Naveed Ayaaz
Nelson, Lancashire, UK
6th Rabi’ al-Awwal, 1435h
7th January, 2014
 Narrated by Abu Sa’eed; collected by Bukhaaree & Muslim
 The strength or weakness of each opinion has not been researched, as the objective behind this article is to only demonstrate the difference of opinion regarding the date.
 This article has been based on research by Abdul’Aleem as-Salafee – May Allaah reward him, For a more in depth discussion: http://www.ahlalhdeeth.com/vb/showthread.php?t=206294