Shab-e Chella(-ye bozorg) ("night of (the great) forty", Persian: شب چلّه) or Shab-e Yaldā ("Yalda night" Persian: شب یلدا) is an Iranian festival celebrated on the "longest and darkest night of the year," that is, in the night of the Northern Hemisphere's winter solstice. Calendarically, 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 (Day) of the Iranian civil calendar.
The longest and darkest night of the year is a time when friends and family gather together to eat, drink and read poetry (especially Hafez) until well after midnight. Fruits and nuts are eaten and pomegranates and watermelons are particularly significant. The red colour in these fruits symbolises the crimson hues of dawn and glow of life. The poems of Divan-e-Hafez, which can be found in the bookcases of most Iranians families, are intermingled with peoples' life and are read or recited during various occasions like this festival and at Nowruz.