"Báb introduced the calendar and its broad pattern of periods and cycles, months and days. Bahá'u'lláh provided essential clarifications and additions. Aspects were elucidated by 'Abdu'l-Bahá, and arrangements for its adoption in the West were put in place at the direction of Shoghi Effendi, as described in the volumes of The Bahá'í World. Still, ambiguities surrounding some Islamic and Gregorian dates, as well as difficulties in the correlation of historical observances and astronomical events with explicit statements in the Text, left certain issues unresolved. When responding to questions concerning the calendar, both 'Abdu'l-Bahá and Shoghi Effendi left these matters to the Universal House of Justice. Of its many features, three required clarification for the calendar's uniform application: the means for the determination of Naw-Rúz, the accommodation of the lunar character of the Twin Holy Birthdays within the solar year, and the fixing of the dates of the Holy Days within the Badí' calendar." [Message from the Universal House of Justice dated 10 July, 2014] (notes below extracted from the message)

The Festival of Naw-Rúz: The birthplace of the Abhá Beauty, will be the spot on the earth that will serve as the standard for determining, by means of astronomical computations from reliable sources, the moment of the vernal equinox in the northern hemisphere and thereby the day of Naw-Rúz for the Bahá'í world.

The Festivals of the Twin Birthdays: They will now be observed on the first and the second day following the occurrence of the eighth new moon after Naw-Rúz. This will result in the observance of the Twin Birthdays moving, year to year, within the months of Mashíyyat, 'Ilm, and Qudrat of the Badí' calendar, or from mid-October to mid-November according to the Gregorian calendar.

The dates of the Holy Days are: Naw-Rúz, 1 Bahá; the Festival of Riḍván, 13 Jalál to 5 Jamál; the Declaration of the Báb, 8 'Aẓamat; the Ascension of Bahá'u'lláh, 13 'Aẓamat; the Martyrdom of the Báb, 17 Raḥmat; the Day of the Covenant, 4 Qawl; and the Ascension of 'Abdu'l-Bahá, 6 Qawl. These dates have been fixed within the solar calendar in accordance with explicit statements of Bahá'u'lláh, 'Abdu'l-Bahá, and Shoghi Effendi. [Message from the Universal House of Justice dated 10 July, 2014]