June 6. On this date in 1892, the Kitáb-i-'Ahd, in which Bahá'u'lláh appoints 'Abdu'l-Bahá as his successor, is read to a large crowd in front of the Tomb of Bahá'u'lláh by Mirza Majdi'd-Din, the grandson of Bahá'u'lláh's only full brother Mírzá Músá and Bahá'u'lláh's scribe.

June 6. On this date in 1892, the Kitáb-i-'Ahd, in which Bahá'u'lláh appoints 'Abdu'l-Bahá as his successor, is read to a large crowd in front of the Tomb of Bahá'u'lláh by Mirza Majdi'd-Din, the grandson of Bahá'u'lláh's only full brother Mírzá Músá and Bahá'u'lláh's scribe.

In the Kitáb-i-'Ahd, Bahá'u'lláh refers to his eldest son 'Abdu'l-Bahá as Ghusn-i-A'zam (meaning "Mightiest Branch" or "Mightier Branch") and his second eldest son Mírzá Muhammad 'Alí as Ghusn-i-Akbar (meaning "Greatest Branch" or "Greater Branch").
Bahá'u'lláh designates his successor with the following verses:

The Will of the divine Testator is this: It is incumbent upon the Aghsán, the Afnán and My Kindred to turn, one and all, their faces towards the Most Mighty Branch. Consider that which We have revealed in Our Most Holy Book: ‘When the ocean of My presence hath ebbed and the Book of My Revelation is ended, turn your faces toward Him Whom God hath purposed, Who hath branched from this Ancient Root.’ The object of this sacred verse is none other except the Most Mighty Branch [‘Abdu’l-Bahá]. Thus have We graciously revealed unto you Our potent Will, and I am verily the Gracious, the All-Powerful. Verily God hath ordained the station of the Greater Branch [Muḥammad ‘Alí] to be beneath that of the Most Great Branch [‘Abdu’l-Bahá]. He is in truth the Ordainer, the All-Wise. We have chosen ‘the Greater’ after ‘the Most Great’, as decreed by Him Who is the All-Knowing, the All-Informed.

This translation of the Kitáb-i-'Ahd is based on a solecism, however, as the terms Akbar and A'zam do not mean, respectively, 'Greater' and 'Most Great'. Not only do the two words derive from entirely separate triconsonantal roots (Akbar from k-b-r and A'zam from *ʿ-z-m), but the Arabic language possesses the elative, a stage of gradation, with no clear distinction between the comparative and superlative.

Majdi'd-Dín was the son of Bahá’u’lláh's full brother Áqáy-i-Kalím, also known as Mirzá Musa. Majdi'd-Dín was married to Samadiyyih, Bahá’u’lláh's daughter from his second wife Fatimih Khanum, who Bahá’u’lláh titled Mahd-i-'Ulya. Both Majdi'd-Dín and Samadiyyih were eventually declared Covenant-breakers for supporting Mírzá Muhammad 'Alí.

Mirza Majdi'd-Din for a time transcribed the Tablets of Bahá'u'lláh, but later became "the most redoubtable adversary of 'Abdu'l-Bahá" by supporting Mírzá Muhammad `Alí, the arch-breaker of the Covenant. Mirza Majdi'd-Din was the one who read the Kitáb-i-'Ahd in front of the family upon the death of Bahá'u'lláh.

Archbreaker of Bahá'u'lláh's Covenant
Announce to National Assemblies that Majdi'd-Din, the most redoubtable adversary of 'Abdu'l-Bahá, denounced by Him as the incarnation of Satan and who played a predominant part in kindling the hostility of `Abdu'l-Hamid and Jamál Páshá, and who was the chief instigator of Covenant-breaking and archbreaker of Bahá'u'lláh's Covenant, and who above sixty years labored with fiendish ingenuity and guile to undermine its foundations, miserably perished struck with paralysis affecting his limbs and tongue. Dispensation of Providence prolonged the span of his infamous life to a hundred years, enabling him to witness the extinction of his cherished hopes and the disintegration with dramatic rapidity of the infernal crew he unceasingly incited and zealously directed, and the triumphant progress and glorious termination of 'Abdu'l-Bahá's thirty-year ministry as well as evidences of the rise and establishment in all continents of the globe of the administrative order, child of the divinely-appointed Covenant and harbinger of the world-encircling order.
--Shoghi
[Cablegram, June 3, 1955]