Shab-e Yalda ("Yalda night" Persian: شب یلدا‎) or Shab-e Chelleh ("night of forty", Persian: شب چله‎) is a Persian festival celebrated on the "longest and darkest night of the year," Yalda is a winter solstice celebration.that is, in the night of the Northern Hemisphere's winter solstice. Calendrically, this corresponds to the night of December 20/21 (±1) in the Gregorian calendar, and to the night between the last day of the ninth month (Azar) and the first day of the tenth month (Dey).