[Extract from a 1965 Ridvan message of the Universal House of Justice announcing plans for a befitting celebration of that historic event:]
The fourth challenge is to prepare national and local plans for the befitting celebration of the centenary of Bahá'u'lláh's proclamation of His Message in September/October, 1867, to the kings and rulers of the world, celebrations to be followed during the remainder of the Nine Year Plan by a sustained and well-planned programme of proclamation of that same Message to the generality of mankind.
A review of the historic proclamation by Bahá'u'lláh, as described by Shoghi Effendi in God Passes By, reveals that its "opening notes" were "sounded during the latter part of Bahá'u'lláh's banishment to Adrianople," and that, six years later, it "closed during the early years of His incarceration in the prison-fortress of 'Akká." These "opening notes" were the mighty and awe-inspiring words addressed by Him to the kings and rulers collectively in the Suriy-i-Mulúk, "the most momentous Tablet revealed by Bahá'u'lláh." It was penned some time during the months of September and October, 1867, and was followed by "Tablets unnumbered ... in which the implications of His newly-asserted claims were fully expounded." "Kings and emperors, severally and collectively; the chief magistrates of the Republics of the American continent; ministers and ambassadors; the Sovereign Pontiff himself; the Vicar of the Prophet of Islam; the royal Trustee of the Kingdom of the Hidden Imam; the monarchs of Christendom, its patriarchs, archbishops, bishops, priests and monks; the recognized leaders of both the Sunni and Shí'ah sacerdotal orders; the high priests of the Zoroastrian religion; the philosophers, the ecclesiastical leaders, the wise men and the inhabitants of Constantinople -- that proud seat of both the Sultanate and the Caliphate; the entire company of the professed adherents of the Zoroastrian, the Jewish, the Christian and Muslim Faiths; the people of the Bayan; the wise men of the world, its men of letters, its poets, its mystics, its tradesmen, the elected representatives of its peoples; His own countrymen"; all were "brought directly within the purview of the exhortations, the warnings, the appeals, the declarations and the prophecies which constitute the theme of His momentous summons to the leaders of mankind ..." "Unique and stupendous as was this proclamation, it proved to be but a prelude to a still mightier revelation of the creative power of its Author, and to what may well rank as the most signal act of His ministry the promulgation of the Kitáb-i-Aqdas." In this, the Most Holy Book, revealed in 1873, Bahá'u'lláh not only once more announces to the kings of the earth collectively that "He Who is the King of Kings hath appeared" but addresses reigning sovereigns distinctively by name and proclaims to the "Rulers of America and the Presidents of the Republics therein" that "the Promised One hath appeared." Such was the proclamation of Bahá'u'lláh to mankind. As He Himself testified, "Never since the beginning of the world hath the Message been so openly proclaimed."
(The Universal House of Justice, ‘Messages from the Universal House of Justice 1963 to 1986’)