Abdu’l-Baha was the son of Baha’u’llah. The day signifies the anniversary of when he passed, beginning at 1 a.m. on Nov. 28. In his eulogy he was described “as One who led humanity to the ‘Way of Truth,’ as a ‘pillar of peace’ and the embodiment of ‘glory and greatness.'"
Sources: https://www.bahaiblog.net/collection/calendar/the-ascension-of-abdul-baha.
