The Festivals of the Twin Birthdays or the Twin Holy Birthdays refers to two successive holy days in the Bahaʼi calendar that celebrate the births of two central figures of the Bahaʼi Faith.
Mirza Husayn-‘Ali, later known as Baha’u’llah, “the Glory of God,” was born in 1817 in Tehran, the capital of Persia, now called Iran.
Two years later, in 1819, Siyyid ‘Ali-Muhammad was born in Shiraz, a city in southern Persia. He later took the title, the Bab, meaning “the Gate.”
The Bab and Bahaʼu’llah are revered as central figures of the Bahaʼi Faith. Both are Manifestations of God in their own right. The lives and missions of the Bab and Bahaʼu’llah are inextricably linked. Together, these Divine Messengers are the twin Manifestations of God for this era.
In 1844, in Shiraz, at age 25, the Bab declared that He had been called by God to be humanity’s gateway to a new and glorious future. As one of God’s Divine Educators, the Bab established a new religion. He called for the people to prepare for the coming of “Him Whom God shall make manifest,” another Divine Messenger Who would soon appear and initiate an era of global justice, unity, and peace. The Bab was an advocate of the poor. He supported the advancement of women, universal education, and the study of science—radical ideas in that time and place.
In April of 1863, Baha’u’llah announced that He was the Promised One foretold by the Bab and all of the Divine Messengers of the past. Baha’u’llah’s Mission was to spiritually re-awaken humanity and unite all the peoples of the world. Through Baha’u’llah God has released a fresh, far-reaching, creative spiritual energy into the world. Baha’u’llah’s teachings shed new light on God’s eternal relationship with humanity and offer a vision of infinite hope and healing.
In the Kitab-i-Aqdas, the central book of the Baha’i Faith, which is the main source of Baha’i law and practices, Bahaʼu’llah wrote that His birthday and that of Bab “are accounted as one in the sight of God.”
Their birthdays are observed on the first and the second day following the occurrence of the eighth new moon after the Baha’i New Year, which is called Naw-Ruz, and occurs on the first day of spring as determined in advance by astronomical tables using Tehran as the point of reference. This results in the observance of the Twin Birthdays moving, year to year, from mid-October to mid-November in the Gregorian calendar.
Sourced from https://www.bahai.us/bahai-house-of-worship-hosts-broadcasts-for-twin-holy-birthdays