The Birth of The Prophet (ṣallallāhu ʿalayhī wa-sallam)
A timely response to those misguided people who innovate into the pure religion of Allāh and form celebrations of the birthday (or what they claim is the birthday) of the Prophet (ṣallallāhu ʿalayhi wa-sallam)
Celebrating the Mawlid has no basis in the Book or the Sunnah, nor is this action recorded from any of the scholars of this Ummah; those who are taken as Imāms to be followed and who adhere to the narrations. Rather it is an innovation which was introduced by the Baṭālūn sect
THE YEAR OF HIS NOBLE BIRTH
According to the most correct opinion of the scholars, the Prophet (ṣallallāhu ʿalayhi wa-sallam) was born in the city of Makkah in the Year of the Elephant (in the year 570 or 571 CE), in the month of Rabīʿ‘ul Awwal. 
THE DAY OF HIS NOBLE BIRTH
There is an agreement amongst the scholars that the Prophet (ṣallallāhu ʿalayhi wa-sallam) was born on a Monday, and he said: "On that day I was born and on that day Revelation descended upon me."  However, as regards to the exact date of his birth, then the scholars have differed about this, although many scholars say he was born on the 12th of Rabīʿul Awwal.
Imām al-Nawawī (d. 676H) - raḥimahullāh - said:
"There is an agreement that he was born on a Monday in Rabīʿ‘ul Awwal. There is a difference of opinion whether that day was the 2nd, 8th, 10th, or 12th of the month - and these are the four most well known opinions concerning this." 
EVENTS AT THE TIME OF HIS NOBLE BIRTH
Certain miraculous events are reported to have occurred at the time when the Prophet (ṣallallāhu ʿalayhi wa-sallam) was born. However, most of them are not authentically related, rather they are da‘eef (weak) or mawdoo‘ (fabricated) and therefore cannot be relied upon as decisive proof; such as the narration which states that some of the galleries of Kisraa's palace broke up and collapsed, that the sacred fire of the Magians died out, and that some of the churches on Lake Saawah collapsed and sank down.  However, it is authentically related that the Prophet (ṣallallāhu ʿalayhi wa-sallam) said: "I am a result of the supplication of my father Ibrāhīm and the glad tiding brought by ʿĪsá (ʿalayhimus salām). And my mother - when she bore me - saw a light shone out from her, which lit up the palaces in Syria…" 
CELEBRATING THE DAY OF HIS NOBLE BIRTH
Imām al-Faakihaanee (d. 734H) - raḥimahullāh - said: 
"Celebrating the Mawlid has no basis in the Book or the Sunnah, nor is this action recorded from any of the scholars of this Ummah; those who are taken as Imāms to be followed and who adhere to the narrations. Rather it is an innovation which was introduced by the Battaaloon sect." 
As regards to the origin of this newly invented celebration, then some of the researching scholars have stated that the first person to innovate this practice was ʿUmar Ibn Muḥammad al-Mulā in the city of Mawsil in ʿIrāq, during the fourth century as is mentioned by Imām Abū Shāmah (d. 665H).  He was followed in his day by the likes of Abū al-Khaṭṭāb ʿUmar Ibn Dihyaa: "who was employed in the west, then traveled to Syria, then he traveled to the city of Irbil in ʿIrān, during the fourth century, where he founds its king, Mudthaffarud-Dīn Ibn Zaynud Dīn showing a keen interest in the milād (birthday) of the Prophet (ṣallallāhu ʿalayhi wa-sallam). So he composed a book for him called 'at-Tanweer Fee Mawlidis Siraajil Muneer'; so he recited this to the king who then awarded him with one thousand dīnārs." 
Imām Mālik (d.179H) - raḥimahullāh - said:
"Whosoever introduces into Islām an innovation, and holds it to be something good, has indeed alleged that Muḥammad (ṣallallāhu ʿalayhi wa-sallam) has betrayed his message. Read the statement of Allāh - the Most Blessed, the Most High:
"This day I have perfected your Religion for you, completed my favour upon you and I have chosen for you Islām as your Religion." [Sūrah al-Maa’idah, 5:3]
So that which was not part of the Religion at that time, cannot be part of the Religion today. And the last part of this ummah cannot be rectified, except by that which rectified its first part." 
Thus, had the practice of celebrating milādun-nabee (the birthday of the prophet) been something praiseworthy, then: "The salaf - may Allāh be pleased with them - would have instituted it. For they were the ones having the greatest honour and love or the Prophet (ṣallallāhu ʿalayhi wa-sallam) and a greater zeal for doing good. Indeed, the most perfect expression of love and honour for him is by following him, obeying him, carrying out his commands, upholding and reviving his Sunnah - both inwardly and outwardly - and spreading his massage and striving in this with the heart and the hand and the tongue. Such was the path of the Companions and those who followed them in goodness." 
 Refer to at-Taareekh (p. 53) of Khaleefah Ibn Khayaat, as-Seerah (1/167) of Ibn Hishaam, also Tabaqaatul Kubraa (1/62) of Ibn Sa'ad.
 Related by Muslim (2/820) and Aḥmad (5/297)
 Tahdheeb Seeratun Nabawiyyah (p. 20) of Imām al-Nawawī.
 Munkar: Related by Imām al-Dhahabī in as-Seeratun Nabawiyyah (p. 11-14) who said: "This narration is munkar gharīb (rejected and strange)."
 Related by al-Ḥākim in al-Mustadrak (2/600) and Ibn Kathīr in al-Bidāyah wan-Nihāyah (1/229) who said: "Its chain of narrators is good and strong." Refer to al-Ṣaḥihah (no. 1545) of Shaykh al-Albānī.
 Al-Mawrid Fee ‘Amalil Mawlid (p. 21-22)
 The Battaaloon: They are one of the deviated Baatiniyyah Shīʿah sects from the Faatimids - as al-Maqreezee says in al-Khatat (1/490).
 al-Bāʿith 'ʿalá Inkaaril Bida‘ wal-Hawaadith (p. 23-24), Ibn al-Jawzee stated something similar in Miraatuz Zamaan (8/310).
 al-Bidāyah wan-Nihāyah (13/144-145) of al-Ḥāfiẓ Ibn Kathīr.
 Related by al-Qaadee 'Iyyaadh in ash-Shifaa fī Huqooqil Musṭafá (2/676).
 Iqtidaa‘us Siraatil Mustaqeem (p. 295) of Shaykh al Islām Ibn Taymīyyah.