Narrators of history suggest four historic events which mark the day of 9th Rabi al-Awwal as significant. Firstly, some historians are of the opinion that the Prophet Muhammad (pbuh) was born on the 9th of Rabi al-Awwal. There are two other opinions: 12th Rabi al-Awwal and 17th Rabi al-Awwal respectively. For the Shia's, the most popular narration is the one which states that the Prophet Muhammad (pbuh) was born on the 17th Rabi al-Awwal 570 AD.
Whatever the case, the date does not make a big difference. Because Ayatullah Ruhullah Musawi al-Khomeini (ra), the founder of the Islamic revolution had declared the week between 9th and 17th Rabi al-Awwal as "Usboo al-Wahda" meaning "the week of unity amongst Muslims." Muslims all over the world are urged to get together and celebrate (Eid-E-Milad un Nabi) the birth of the Prophet Muhammad (pbuh) for the whole week.
Secondly, in regards to the importance of this day, it is noted that the Prophet Muhammad (pbuh) himself was seen smiling and 'celebrating' once in the gathering in the city of Madinah on this day while in the presence of Imams Ali, Imam Hassan and Imam Hussain (as) and said, "It is on this day that Allah (SWT) will destroy your enemies and the enemies of your grandfather and it is on this day when Allah (SWT) will accept the actions of your Shia and those who love you. This is the day when the words of Allah (SWT) came true where He said (in the Glorious Qur'an): 'So those are the houses fallen down because they were unjust ...' (27:52). And this is the day when the Pharaoh (Firawn) of the time of the Ahlul Bayt was destroyed ..."
Thirdly, the 9th of Rabi al-Awwal is also known as Eid-E-Zahra meaning the festival of Fatima al-Zahra (sa), because 3 to 4 years after the tragedy of Karbala, it was on this day that joy and happiness was restored in the family of Ahlul Bayt (as). From the tragedy of Karbala in 61 AH till this day, the family members of the Prophet Muhammad (pbuh) had continuously mourned and grieved the martyrdom of Imam Hussain (as).
Then Mukhtar ibn Ubaydullah al-Thaqafi, together with Ibrahim ibn Malik al-Ashtar and others rose to avenge the blood of Imam Hussain (as). They arrested every killer of karbala and executed them. Finally, Mukhtar arrested Umar ibn Saad ibn abi Waqqas (commander-in-chief of Yazeed's army at Karbala) and Ubaydullah ibn Ziyad (governor of Yazeed in Kufa). He beheaded them and sent their heads to Imam Ali ibn al-Hussain al-Zainul Abideen (as) in Madina. These two heads reached the Imam Ali ibn al-Hussain al-Zainul Abideen (as) on the 9th Rabi al-Awwal and on seeing them, Imam (as) went into prostration (Sajda-E-Shukr) and said, "I thank Allah (SWT) who kept me alive to this day so that I could see the heads of the tyrants who killed my father." Imam Ali ibn al-Hussain al-Zainul Abideen (as) then instructed the members of his family to remove the clothes of mourning, adorn themselves and celebrate the day with joy and happiness.
Umar ibn Saad ibn abi Waqqas (la), the killer of Imam Hussain (as) was the wretched criminal who was the first one to shoot an arrow towards Imam Hussain (as) on the 9th of Muharram al-Haram 61 AH, thus formally starting the war against Imam Hussain (as)! And he proclaimed, "O people of Kufa and Shaam, be witness hereto on the Day of Judgment, that I am the one who is the first to shoot an arrow at Hussain"! And saying this, he shot the arrow! Later, Imam Hussain (as) told them [the army of Yezeed whose commander-in-chief was this Umar ibn Saad ibn abi Waqqas (la)] after delivering an eye-opening Khutbah (sermon), that he needs to have a night for prayers!
Mukhtar ibn Ubaydullah al-Thaqafi announced the aims of his revolution in Masjid-E-Kufa, "I will conform to Allah's Book and to His Prophet's Sunnah. I will take the revenge on Imam Hussain's killers. I will fight against those who have broken Allah's laws. I will defend the weak against the strong." It is only the lucky ones who have the opportunity to celebrate this Eid! This was the day; when for the first time after the massacre of his father Imam Hussain (as) and his companions that our 4th Imam, Imam Ali ibn al-Hussain al-Zainul Abideen (as) smiled! Is this not enough an excuse to celebrate? For, our joys are with the joys of Masoomeen (infallible) and our grief and sorrows are for them alone!
Hence, 9th of Rabi al-Awwal is the day of glory, honour, dignity and blessings. It is a great Eid day also named as "Eid-E-Shuja."
Fourth or finally, 9th Rabi al-Awwal is the first day of the Imamate of our living Imam, al-Hujjat ibn Hassan al-Askari [may Allah (SWT) hasten his return] - a day of rejoicing and celebration.
It is true that an Imam is an Imam from birth but every Imam assumed the official role of his Imamate after the death of the previous Imam. The reason why we celebrate, in particular, the Imamate of our living Imam, al-Hujjat ibn Hassan al-Askari [may Allah (SWT) hasten his return] is to perhaps remind ourselves of his presence and about our responsibilities towards him. His father and our 11th Imam, Imam Hassan Askari (as) passed away on the 8th of Rabi al-Awwal 260 AH and our 12th Imam Muhammad al-Mahdi [may Allah (SWT) hasten his return] assumed his Imamate formally at a young age of 5 years on the 9th Rabi al-Awwal 260 AH.
We pray to Allah (SWT) to keep us firm on His path, and to always protect us from negligence in discharging our duties towards Him and our living Imam Muhammad al-Mahdi [may Allah (SWT) hasten his return], and to also include us among the Imam Muhammad al-Mahdi [may Allah (SWT) hasten his return] helpers and companions - Elahi Ameen.
One of the ways which we can strive to be amongst the companions of our living Imam Muhammad al-Mahdi [may Allah (SWT) hasten his return] is to follow the responsibilities which rest upon us, including the recitation of Dua al-Ahd (the Pledge) every morning in which we proclaim: "O' Allah, truly I renew on the dawn of this day and all the days of the remainder of my life my pledge, covenant and allegiance to him, as my obligation, which I shall neither contravene, nor neglect ... O' Allah include me among his helpers, defenders, those fulfilling his wishes and commands, his defenders, the fore-runners to carry out his intentions and those to be martyred in his presence ... O' Allah: make him the refuge for Your oppressed servants; a helper for him who has no other helper besides You; the reviver of the commandments of Your book which have been neglected; the strengthener of the sciences in Your religion, and the traditions of Your Prophet. May Allah's blessing be upon him and his progeny."
Hadith about Imam Muhammad al-Mahdi [may Allah (SWT) hasten his return]:"The world will not come to an end," said the Prophet Muhammad (pbuh), "until a man from my family (Ahlul Bayt) and of my name shall be master of the world, When you see a green ensign coming from the direction of Khorasan, then join them, for the Imam of God will be with the standards who will be called Al-Mahdi."
Last but not least: Ahmad ibn Ishaq al-Qummi was a great companion of our 11th Imam, Imam Hassan Askari (as). On the 9th Rabi al-Awwal, Muhammad Hamadani and Yahya Baghdadi came to visit him and were informed that Ahmad ibn Ishaq al-Qummi was busy performing Ghusl (Ablution) for that day. When they inquired what the Ghusl was about, they were told that Ahmad ibn Ishaq al-Qummi had heard from 10th Imam, Imam Hadi (as), who said: "9th of Rabi al-Awwal is the day of Eid. It is our greatest Eid and the Eid of our followers."
After performing his Ghusl Ahmad ibn Ishaq al-Qummi said to the two visiting men: "I have performed Ghusl because today is the Eid of 9th Rabi al-Awwal. I visited Imam Hassan Askari (as) on this day and noticed that his ring was shinning. The people of his household wore new clothes and had applied perfume. When I asked the reason, Imam Hassan Askari (as) said, 'Today is 9th Rabi al-Awwal. It is the day of Eid for us and for our followers'". (Chauda Sitaare)