USBN #122 January 1939 p3
"The Baha'i day starts and ends at sunset, and consequently the date of the celebration of Baha'i feasts should be adjusted to conform to the Baha'i calendar time. For further particulars on this subject you should refer to the section entitled 'Baha'i Calendar' in The Bahai World.
"The Guardian would advise that, if feasible, the friends should commemorate certain of the feasts and anniversaries at the fol- lowing time:
"The anniversary of the Declaration of the Báb on May 22, at about two hours after sunset.
"The first day of Ridván, at about 3:00 P.M. on the 21st of April.
"The anniversary of the Martyrdom of the Báb on July 9 at about noon.
"The anniversary of the Ascension of Baha'u'llah on May 29 at 3:00 A.M.
"The Ascension of 'Abdu'1-Bahá on November 28 at 1:00 A.M.
"The other anniversaries the believers are free to gather at any time during the day which they find convenient."
Letter from Shoghi Effendi, November 27, 1938.