Shab-e Yalda "Yalda night” or Shab-e Chelleh "night of forty" 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.
In the Zoroastrian era, people were advised to stay awake for most of the night, to prevent any bad luck from happening. People had to gather in the safety of groups of friends and relatives, share the last remaining fruits from the summer, and find ways to pass the long night together in good company.
This same culture still exist. Every Yalda night people are gathering with their loved ones. They would especially go to their grandparents’ house. Eat, drink and read poetry(Hafez) during the whole the evening to pass the darkest evening of the year with laughter and joy. Iranians believe those who begin winter by eating summer fruits would not fall ill during the cold season. Honestly this evening is filled with different type of food. Gaining weight is very common in that evening!
The most important fruits of the evening are pomegranate, the jewel of all fruits, and watermelon. Without them, there is no Yalda night. The color of these fruits symbolise the cycle and glow of life. These days you can find everything in the shape and color of these fruit such as cake, cupcakes or even in home accessories.
As I mentioned, during this night people also read Hafez poetry. You probably wonder why Hafez Iranians believe in Hafez as much as believing in God. People make a wish, open a book of Hafez and the first poem they see is the interpretation of the wish and whether and how it will come true. So in Yalda night each member of the family makes a wish and randomly opens the book and asks the eldest member of the family to read it aloud.