The Ninth of Rabiʿ al-Awwal; a day of rejoicing and celebration
9th Rabi al-Awwal is the first day of the Imamate of our living Imam, Imam al-Mahdi; a day of rejoicing and celebration. It has been narrated that to give alms on this day brings about forgiveness of sins. It has been also said that it is recommended, on this day, to offer food, give pleasure to one's brethren-in-faith, confer upon one's family members with gifts, new clothes, and all that they need, praise and thank Almighty Allah, and practice acts of worship to Him. This day, which is also a holy day, is thus the day of removing grievances and sorrows. As the past day was the day of Imam Hassan Askari's martyrdom, the ninth of Rabi al-Awwal is thus the first day in the Imamate of Imam Mahdi (A.S), may Allah hasten his advent. This is an additional point of happiness on this day.
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.
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.
Source: https://hadith.net/en/post/55030/the-ninth-of-rabi-al-awwal/