TEHRAN – The Shia Muslims across the world, including in Iran, celebrated the Eid al-Ghadir, which marks the appointment of the first Shiite Imam, Ali ibn Abi Talib (AS), by Islam’s most revered figure, Prophet Muhammad (PBUH), as his successor.
Great masses of people in the Iranian capital of Tehran have marked Eid al-Ghadir by attending the 10-kilometer-long party which has been hailed as the world’s largest celebration event of its kind 
The event is celebrated annually around the world by Shia Muslims. In Iran, Iraq, Afghanistan, Pakistan, Turkey, Azerbaijan, Yemen and several other countries masses celebrated Eid al-Ghadir on the 18th of Dhul-Hajjah. 
On this day different programs and events such as musical performances and plays are carried out and food and drinks are distributed are handed out as part of the massive celebration.
In the Iranian capital of Tehran, a grand ceremony organized in a route extending for 10 kilometers between the Azadi (liberty) square and the Imam Ali highway.
The streets were decorated with green flags and other decorative elements as part of the celebrations marking the event. The preparations for the major event had started a few weeks earlier. 
On the auspicious occasion, people came to streets and extended greetings and felicitations to each other, especially to the families who are descendants of Prophet Muhammad (PBUH) through his grandsons, Hassan ibn Ali (AS) and Hussein ibn Ali (AS), the sons of the Prophet’s daughter, Fatima Zahra (SA), and his son-in-law Ali ibn Abi Talib (AS).
Many young Iranian couples choose the auspicious event as their wedding date in the hope that their marriage is blessed with the memory of Imam Ali (AS).
According to the Holy Qur’an [5:3], this is the day when God Almighty “perfected” Islam as the religion by introducing Imam Ali (AS) as the next leader. That is why it is celebrated in honor of the first Shia Imam.  
Eid al-Ghadir is also referred to as Eid al-Akbar (the Greatest Eid) in Islamic narrations, since Muslims are of the opinion that the mission of all the previous messengers of Allah became accomplished on this day.
Ali (AS) was the cousin of the Prophet (PBUH) and is known as the first man to embrace the Prophet’s call for Islam. According to Islamic sources, the Imam rooted out poverty and promoted an unprecedented justice system across the Muslim world during his 5-year rule-based in the Iraqi city of Kufa.
There are 12 Imams or religious leaders in Shia Islam, all believed to be preserving the legacy of the Prophet (PBUH) and his teachings. Mahdi (AS) is the last Imam, who is believed to be the apocalyptic savior of the world and is currently living in occultation.