March 19

The Naw-Ruz Feast should be held on March 21 before sunset and has nothing to do with the Nineteen Day Feast. The Nineteen Day Feast is administrative in function whereas the Naw-Ruz is our New Year, a Feast of hospitality and rejoicing.
(In a letter written on behalf of Shoghi Effendi, 5 July 1950 to the National Spiritual Assembly of the United States and Canada; The Compilation of Compilations, vol. I, The Nineteen Day Feast)