Yalda night or Shab-e Chelleh is an Iranian 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 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) of the Iranian civil calendar.