It is written in the Most Holy Book (p.254) that: …the Festival of Riḍván, which commemorates Bahá’u’lláh’s Declaration of His Prophetic Mission in the Garden of Riḍván in Baghdád during twelve days in April/May 1863 and is referred to by Him as “the King of Festivals”.
Bahá’u’lláh spent most of His Blessed life in prison and exile. When He got exiled once again, He stayed for 12 days in a garden outside Baghdad. This garden is known as the Ridvan garden. It was there where He announced to those present that He was the One, the Messenger of God they had all been waiting for, and the One that The Báb had been preparing them for. Here is a more accurate explanation by Shoghi Effendi in the book God Passes by (p.412):
The arrival of Bahá’u’lláh in the Najíbíyyih Garden, subsequently designated by His followers the Garden of Riḍván, signalizes the commencement of what has come to be recognized as the holiest and most significant of all Bahá’í festivals, the festival commemorating the Declaration of His Mission to His companions. So momentous a Declaration may well be regarded both as the logical consummation of that revolutionizing process which was initiated by Himself upon His return from Sulaymáníyyih, and as a prelude to the final proclamation of that same Mission to the world and its rulers from Adrianople.
All the Messengers of the past had been announcing His coming and in diverse Holy Scriptures we can find mention of Bahá’u’lláh’s Dispensation. For instance, Jesus Christ said He would return in the Name f the Glory of the Father – see how Bahá’u’lláh means “The Glory of God”. Shoghi Effendi in God Passes By (p.412) wrote:
He Who in such dramatic circumstances was made to sustain the overpowering weight of so glorious a Mission was none other than the One Whom posterity will acclaim, and Whom innumerable followers already recognize, as the Judge, the Lawgiver and Redeemer of all mankind, as the Organizer of the entire planet, as the Unifier of the children of men, as the Inaugurator of the long-awaited millennium, as the Originator of a new “Universal Cycle,” as the Establisher of the Most Great Peace, as the Fountain of the Most Great Justice, as the Proclaimer of the coming of age of the entire human race, as the Creator of a new World Order, and as the Inspirer and Founder of a world civilization.
To Israel He was neither more nor less than the incarnation of the “Everlasting Father,” the “Lord of Hosts” come down “with ten thousands of saints”; to Christendom Christ returned “in the glory of the Father,” to Shí’ah Islám the return of the Imám Ḥusayn; to Sunní Islám the descent of the “Spirit of God” (Jesus Christ); to the Zoroastrians the promised Sháh-Bahrám; to the Hindus the reincarnation of Krishna; to the Buddhists the fifth Buddha.
Baha’is all over the world not only celebrate the 20th or 21st of April as the beginning of this 12 day festival but the 9th day and the 12th day also are observed as Holy Days which means we take permission from work/school and celebrate this momentous occasion. Furthermore, it is then when the Local and National Spiritual Assemblies are elected every year and when the Universal House of Justice is also elected.
So, if you are a Baha’i, you are already celebrating. If not, join us in celebration!! For this is a day of joy and gratefulness to God for all the bounties He constantly showers upon us.