Before his death in 1892, Baha'u'llah composed a document known as the Kitab-i-Ahd, the Book of the Covenant, in which he formally appointed his eldest son, Abbas Effendi, later known as Abdu'l-Baha, meaning Servant of Baha, as the authorized interpreter of his writings and the head of the Baha'i Faith following his own death. The document was read aloud to family members and close followers shortly after Baha'u'llah's passing, establishing a clear and, within Baha'i teaching, divinely sanctioned line of succession at a moment when the still young religious movement might otherwise have faced significant uncertainty over leadership.
Abdu'l-Baha went on to lead the Baha'i community for nearly three decades, overseeing its expansion beyond the Middle East into Europe and North America through his own travels and through correspondence, and producing an extensive body of his own writings and talks that, while not accorded the same scriptural status as Baha'u'llah's own revelation, are treated within the Baha'i Faith as authoritative interpretation of it.
The appointment is commemorated annually on 26 November as one of the days within the Baha'i calendar associated with the Covenant, distinct from, though related to, the nine principal holy days on which work is suspended.