Directives From the Guardian
Page 30
 

76: NAW-RÚZ, TIME OF CELEBRATION OF
 
1

"Regarding Naw-Rúz: If the vernal equinox falls on the 21st of March before sunset, it is celebrated on that day. If at any time after sunset, Naw-Rúz will then, as stated by Bahá'u'lláh, fall on the 22nd. As to which spot should be regarded as the standard, this is a matter which the Universal House of Justice will have to decided. The American NSA need not therefore take any action in this matter at present."