Eid-e-Shuja', also known as Eid-e-Zahra, is a ritual festival observed by most Twelver Shi‘a Muslims and some groups of Sufis. It marks the end of the two-month mourning period (azadari) after the events of the Karbala massacre, which occurred in 680 AD (61 AH). It is celebrated annually on the ninth day of the month of Rabi Al-Awwal in the Islamic-Hijri calendar.
Eid is an Arabic word that is often translated as "festivity", while the term Shuja' means "brave" or "courageous". Eid-e-Shuja’ stands for "eid of the brave and courageous companions of Imam Hussain ibn Ali" who fought in the battle of Karbala. In this uneven battle, the Ommayad army of Yazid I slaughtered all of Hussain's able male companions, with Hussain, according to Shia tradition, the last to be killed. The Ommayads looted Hussein's camp, decapitated the bodies of his companions, and took as prisoners all the women and children, amongst them was Hussein's surviving son ʿAlī, who became the fourth Shiʿite Imam, Zayn-al-ʿAbedin.
According to some Shi'a traditions, this is the day when the decapitated heads of the key murderers in the Karbala massacre, namely Umar ibn Sa'ad and Ubayd-Allah ibn Ziyad, who were killed by Mukhtar al-Thaqafi (d. 687), reached Imam Zayn-al-'Abedin in Madina and brought a smile to his face.
Eid-e-shuja is also commonly referred to as Eid-e-Zahra (“Eid of the family of Fatimah”), who was a daughter of the Prophet Muhammad and mother of Imam Hussein. In this way the celebration of Eid-e-Shuja is also a veneration of the day her son's murderers were caught and is considered an occasion of great exultation and happiness.
Death of Imam al-Askari
The two-month period of Azadari (mourning) reaches its conclusion with the assassination of the eleventh Shi’a Imām, Hasan al-Askari on the eight of Rabi al-Awwal 260 AH (approximately 4 January 874). This date signals the closure of a long mourning season during which bright clothes, extravagant purchases and overt celebrations are all considered inappropriate. Pious Shias are now allowed to resume their less restricted lifestyles, following a tradition that holds that the Prophet's family itself shed its mourning garb on the ninth.
Another primary religious event which is celebrated on the eid-e-shuja’ is the believed beginning of the Imamate of the twelfth Shī‘a Imām, Muhammad al-Mahdi. However, this assertion remains highly disputed among several various Shī‘a sects, such as the Ismā'īlī and Zaydī Shī'ī denominations, who each believe in a different number of Imāms and, for the most part, a different path of succession regarding the Imāmate. This is also disputed because there is no consensus on the exact date of martyrdom of Imam Hasan Al-Askari in the Twelver Shia literature.  Some scholars believe that 9th Rabi ul Awal is the second day of the Imamat of Imam Al-Mahdi since the eleventh Imam passed away on dawn of the 8th of Rabi ul Awal. Since the succeeding Imam becomes an Imam the moment that the previous Imam dies, Eid e Zahra would be the second day of the Imamate of Imam Al-Mahdi.