According to Wikipedia Yaldā Night (Persian: شب یلدا‎ shab-e yalda) or Chelleh Night (Persian: شب چله‎ shab-e chelle) is an Iranian Northern Hemisphere's winter solstice festival celebrated on the "longest and darkest night of the year." 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.