Just like the dates of birth and death of the Holy Prophet the date of his appointment to the prophetic mission is also not known definitely from the historical point of view; the Shi'ah scholars are almost unanimous that he was appointed to the prophetic mission on the 27th of Rajab and his prophethood commenced from the same day. However, the Sunni scholars usually claim that he was appointed to this mission during the holy month of Ramadan.
As Shi'ahs claim to be the followers of the progeny of the Prophet and consider their reports to be true and final in the light of 'Hadith Thaqalayn', they have accepted the report of the progeny of the Prophet's Household about the date of this appointment to the prophetic mission to be correct. The descendants of the Prophet say: "The chief of the family was appointed to the prophetic mission on the 27th of Rajab". In view of this, Shi'ahs do not entertain any doubt about the correctness of this date.
The thing which can be treated to be the basis for the other view is the very statement of the Holy Qur'an, wherein it has been stated that the verses of the Qur'an were revealed in Ramadan. And as the day of the commencement of the prophetic mission is the day of the beginning of revelation of the Holy Qur'an it may be said that the Prophet was appointed to prophethood in the very month of Ramadan. The verses of the Holy Qur'an which indicate that it was revealed in Ramadan are as follows:
And that is the very night of Qadr (glory) which has been mentioned in Surah al-Qadr (verse 1) wherein it has been said: "We revealed the Qur'an in the Night of Glory."
REPLY BY SHI'AH SCHOLARS
The Shi'ah traditionalists and commentators have given various replies and explanations with regard to this argument; some of them are mentioned below:
THE FIRST REPLY
The above-mentioned verses show that the Holy Qur'an was revealed in one blessed night of the Holy month of Ramadan, which is called the 'Night of Glory', but they do not mention the place of its revelation and do not also denote that they were revealed to the Prophet during the same night. It is quite possible that there may have been various revelations of the Qur'an and numerous Shi'ah and Sunni narrations confirm this possibility. One of those revelations is the gradual revelation of the Qur'an to the Prophet and the other is its revelation in installments from the Lauhul Mehfuz  to Baytul Ma'mu'r.  Hence there should be nothing wrong in a few verses of Surah al-Alaq being revealed to the Holy Prophet on the 27th of Rajab and the entire Qur'an, in one compact form, being revealed in the month of Ramadan from a place named by the Qur'an as 'Lauhul Mehfuz' to another place 'Baytul Ma'mur.' This view is confirmed by the very verse of Surah al-Dukhan which says: We swear by the glorious Book that We revealed the Qur'an on a blessed night. It is evident from this verse (on the strength of the pronoun which turns towards the word 'Book') that it was revealed in its entirety during the Glorious Night (falling in the month of Ramadan), and it is only proper that this revelation should be other than the revelation which took place at the time of the appointment of the Prophet to the prophetic mission, because at that time only a few verses were revealed.
In short the verses which indicate that the Qur'an was revealed in the month of Ramadan during the 'Night of Glory' cannot be a proof of the fact that the day of the Holy Prophet's appointment to the prophetic mission, on which only a few verses were revealed, also coincided in the same month, because the aforesaid verses show that the entire Book (the Qur'an) was revealed in that month, whereas, on the day of the Holy Prophet's appointment to the prophetic mission only five or six verses were revealed. Hence, the two schools, while explaining the meanings of the above-mentioned verses in their narrations, say that the entire Qur'an was sent down from the 'Lahhul Mehfuz' to 'Baytul Ma'mur in that very month. The Shi'ah and Sunni scholars have quoted the said narrations and in particular Professor Muhammad Abdul Azim Zarqani of 'Al-Azhar' university has mentioned them in his book in detail.
THE SECOND REPLY
The most sound reply, which has so far been given by the scholars, is the second one. The learned scholar Tabatabai has explained it in detail in his precious book  and a gist of it is given below:
The real and actual purpose of the Holy Qur'an saying that: We revealed it in the month of Ramadan is that it was revealed to the Holy Prophet during that month, because besides being revealed in parts the Holy Qur'an possesses a reality about which Almighty Allah informed His great Messenger in a particular night of the month of Ramadan.
As the Holy Prophet possessed knowledge of the entire Holy Book, he was directed not to make haste in divulging its contents and to wait till he received orders regarding its communication to the people gradually. The Holy Qur'an says: Do not make haste in narrating the contents of the Qur'an till you receive orders in this behalf through revelation.
This reply shows that the Holy Qur'an possesses a total existence which was revealed in its entirety at one time in the month of Ramadan and it also possesses a gradual existence which lasted from the day of the Holy Prophet's appointment to the prophetic mission up to the end of his life.
THE THIRD REPLY
As has been explained briefly regarding the different kinds of revelation, it has evidently various stages. The first stage is that of true visions and dreams. The second stage is that of oracular and heavenly voices which are heard without facing an angel. And the last stage is that when the Prophet sees an angel and hears the words of Allah from him and learns through him about the realities of other worlds.
As the human soul does not possess the requisite strength to bear the different stages of revelation in the very first instance, it is necessary that the capacity to bear them should develop gradually, it may be said that: on the day of his appointment to the prophetic mission (27th of Rajab) and for some time thereafter the Holy Prophet heard only those Heavenly voices which informed him that he was the Messenger and Prophet of Allah and no verse was revealed during that period. And some time later gradual revelation of the Qur'an began in the month of Ramadan.
By this we mean that the appointment of the Prophet to the prophetic mission in the month of Rajab is not attached to the revelation of the Qur'an in that month. In view of this there should be no discrepancy between the Qur'an being revealed in the month of Ramadan and the Prophet being appointed to the prophetic mission in the month of Rajab.
Although what has been stated above does not conform to numerous texts (because many historians have said explicitly that the verses of Surah al-Alaq were revealed on the very day of the appointment of the Holy Prophet to the prophetic mission), still we possess some narratives, which tell us that on the day of the Holy Prophet's appointment to the prophetic mission he heard oracular voice, and which do not say anything about the revelation of the Holy Qur'an or its verses. They explain the matter by saying that on that day the Holy Prophet saw an angel who said to him "O Muhammad! you are the Messenger of Allah ". And in some of the narrations only hearing of the voice by the Prophet has been mentioned and nothing has been said about his seeing the angel.
THE FAITH OF THE PROPHET
For quite some time the question of the religion of the Holy Prophet before his appointment to the prophetic mission has been a matter of discussion between the Shi'ah and Sunni scholars. They have raised the following issues and given reply to each one of them
These are the four questions which one comes across in various books of Seerah (the Prophet's biography), history, and exegesis. However, is it necessary that we should give definite replies to these questions? Is it basically necessary that we should study various books on history, exegesis and the Holy Prophet's biography and collect the requisite answers?
We feel that discussion of these particular points is not at all necessary. Rather what is important is that we should conclusively prove that before his appointment to the prophetic mission the Holy Prophet believed in and worshipped only Allah the One, and was pious and chaste.
This can be proved by the following two methods:
Firstly by studying his forty years' life preceding his appointment to the prophetic mission, and secondly by scrutinizing what has been said on the subject by the leaders of Islam.
1. To sum up his forty years' life was a life of modesty and chastity, honesty and truthfulness, uprightness and righteousness, goodness and kindness to the oppressed and the needy and hatred for the idols and the idol-worshippers. So much so that once. when he performed a journey to Syria in connection with trade and the other party, to a transaction, swore by idols he said "The most obnoxious things which always arouse my wrath are these very 'Lat' and 'Manat' by which you are swearing".
Besides this he prayed continuously in the cave of Hira during the month of Ramadan and performed Haj time and again, for as the sixth Imam says, he (the Holy Prophet) performed Haj secretly ten times and according to another narration twenty times. And, as we know, all Haj ceremonies are the performance of rites to which Prophet Ibrahim invited people and wished that by this means those who believe in Allah the One should assemble at one particular place during a fixed period.
Similarly the Prophet always remembered Allah while taking his meals and refrained from eating meat of the animals which had been slaughtered in an unlawful manner, and was very much disturbed to see obscene scenes, wine-drinking and gambling; so much so that at times he sought asylum in the mountains and returned home when a part of the night had passed. Now what demands our consideration is this:
Is it possible to doubt the faith of a man who has passed his life in the manner stated above and who does not possess even the smallest weak point from the very beginning of his life, and who passes a part of his life in hills and in secluded places to meditate upon the beautiful phenomena of the world. \\'e consider an ordinary man to be pious, dutiful and righteous if we observe one tenth of these qualities in him, what to speak of the Holy Prophet?
2. The second method of finding out the real position is the study of numerous documents and narratives which have reached us from the leaders of Islam. One of them is the discourse of Imam Ali, the chief of the monotheists, in 'Khutbah-i Qasi'ah':
"From the time the Holy Prophet had been weaned, the Almighty Allah had associated the most distinguished angel with him so that he might show him the path of magnanimity and goodness during day as well as night" 
COMPARISON OF PROPHET MUHAMMAD
There is no doubt about the fact that the Holy Prophet of Islam enjoys superiority over all previous Prophets. And it has been stated explicitly in the Holy Qur'an about some Prophets that they were elevated to the position of prophethood during their very childhood and Books were also sent to them. For example, the Holy Qur'an says about Prophet Yahya:
To Yahya we said: Observe the Scripture with a firm resolve. We bestowed on him wisdom, grace and purity while he was yet a child. (Surah Maryam, 19:12)
When Prophet 'Isa was in his cradle the elders of Bani Israel pressed his mother to let them know the father of the child. Maryam pointed towards the cradle so that people should get the reply to their question from the child himself. 'Isa replied to them in these words with great eloquence and firmness: I am the servant of Allah. He has given me the Injeel and ordained me a Prophet. His blessing is upon me wherever I go, and He has commanded me to be steadfast in prayer and to give alms to the poor as long as I shall live. (Surah Maryam, 19:31)
The son of Maryam clarifies for us the principles and regulations of his religion during his very infancy and declares that he follows the law of the worship of Allah, the One. Now we call your conscience to witness and ask you: "When Prophet Yahya and Prophet 'Isa are true believers and pronounce the realities of nature, can it be said that the unique leader of mankind and the noblest of men did not possess an extraordinary faith till the age of forty years, although even at the time of the revelation in the cave of Hira he was already busy in meditation?" See: Allamah Muhammad Husayn Tabatabai, Tafsirul Mizan.
 Manahilul 'Irfan fi 'Ulumil Qur'an, vol. I, page 37.
 al-Mizan, vol. II, pp. 14-16
 Biharul Anwar, vol. XVIll, pp. 184,190 and 193.
 Biharul Anwar, vol. XVIII, pp. 271 - 281.
 Nahjul Balaghah, vol. II, page 182.