"All Feasts have attained their consummation in the two Most Great Festivals, and in the two other Festivals that fall on the twin days." -- Baha'u'llah
This passage establishes four great festivals of the Bahá'í year. The two designated by Bahá'u'lláh as "the two Most Great Festivals" are, first, the Festival of Ridvan, which commemorates Bahá'u'lláh's Declaration of His Prophetic Mission in the Garden of Ridvan in Baghdad during twelve days in April/May 1863 and is referred to by Him as "the King of Festivals" and, second, the Báb's Declaration, which occurred in May 1844 in Shiraz. The first, ninth and twelfth days of the Festival of Ridvan are Holy Days, as is the day of the Declaration of the Bab.
The "two other Festivals" are the anniversaries of the births of Bahá'u'lláh and the Bab. In the Muslim lunar calendar these fall on consecutive days, the birth of Bahá'u'lláh on the second day of the month of Muharram 1233 A.H. (12 November 1817), and the birth of the Bab on the first day of the same month 1235 A.H. (20 October 1819), respectively. They are thus referred to as the "Twin Birthdays" and Bahá'u'lláh states that these two days are accounted as one in the sight of God. He states
that, should they fall within the month of fasting, the command to fast shall not apply on those days. Given that the Bahá'í calendar is a solar calendar, it remains for the Universal House of Justice to determine whether the Twin Holy Birthdays are to be celebrated on a solar or lunar basis.
(The Kitab-i-Aqdas, p. 224)
/// The Bab was born Siyyid 'Ali-Muhammad on October 20, 1819, in Shiraz, Iran.
(NSA USA - Developing Distinctive Baha'i Communities)
/// Other Holy Days -- Naw-Ruz, Ridvan, the anniversaries of the Birth of the Bab and Bahá'u'lláh, and the Declaration of the Bab -- are festive. The general character of the festive Holy Days is described by 'Abdu'l-Bahá in a talk about the Feast of Naw-Ruz:
/// All should rejoice together, hold general meetings, become as one assembly, so that the national oneness, unity and harmony may be demonstrated in the eyes of all.
/// As it is a blessed day it should not be neglected, nor deprived of results by making it a day devoted to the pursuit of mere pleasure.
/// During such days institutions should be founded that may be of permanent benefit and value to the people. . . .
/// Preparations for a Holy Day might include a review of the history of the day and study of the lives of the Central Figures. Before the event itself, the Assembly may want to help the community prepare for a Holy Day by:
/// * Holding a special class on the history of the Holy Day /// * Reviewing the events being commemorated /// * Holding a special meeting for the children so that they can anticipate the day and learn about its significance /// * Discussing plans at preceding Feasts for commemorating the day
When to Observe Definition of the Bahá'í Day
With reference to your question in connection with the observance of Bahá'í Holy Days; the Bahá'í day begins and ends at sunset. The night preceding a Holy day is therefore included in the day, and consequently work during that period is forbidden.Shoghi Effendi, in Dawn of a New Day, p. 68