The believers in the West, at the time of ‘Abdu’l-Baha, wanted to commemorate His birthday that took place on the same day that the Bab declared His Mission. ‘Abdu'l-Bahá, however, didn’t approve of this and told them that this day was not, under any circumstances, to be celebrated as His day of birth and that it should be exclusively associated with the Bab. But as the Bahá'ís begged for a day to be celebrated as His, He gave them November 26th, to be observed as the day of the appointment of the Centre of the Covenant. It was known as Jashn-i-A'zam (The Greatest Festival), because He was Ghusn-i-A'zam -- the Greatest Branch. In the West it is known as the Day of the Covenant.
(Adapted from the book, The Centre of the Covenant by Hand of the Cause Balyuzi)