Yawm‐e Ali – literally the “Day of Ali” – marks the birthday of Hazrat Ali (alayhi salam), the cousin and son-in-law of Prophet Muhammad (peace be upon him and his family), and who, in accordance with Shia Muslim tradition and interpretation of history, is the first in a line of hereditary Imams from the family of the Prophet. According to tradition, Hazrat Ali was born on 13 Rajab in the year 599 CE. It is reported that on this day, Hazrat Ali's mother visited the holy Ka‘ba to pray, and it was there that he was born.
In both Muslim tradition and modern scholarship, Hazrat Ali is recognised as one of the most important spiritual and intellectual authorities in Islam after the Holy Prophet. He is also revered by all Muslims as the last of the four “rightly‐guided caliphs” (al-khulafa al-rashidun).
As caliph and as Imam, Hazrat Ali established a paradigm of Muslim leadership centred on the ethics of Islam, and principles of just and moral leadership. He elucidated the notion of the intellect as a facet of faith that is to be developed and used in keeping with the ethical imperative of Islam, and which is intimately connected with the spirituality of the faith.
In the Shia interpretation of Islam, Allah commanded Prophet Muhammad to appoint Hazrat Ali as his successor. Hazrat Ali and the Imams that followed him are responsible for continuing the teaching and interpretation of God’s final message to mankind, which was revealed to Prophet Muhammad and recorded in the Holy Qur’an.
Reference: https://the.ismaili/global/our-community/festivals/yawm-e-ali