Yalda is one of the most celebrated traditional events in Iran which marks the longest night of the year, that is, in the night of the Northern Hemisphere's winter solstice.
Ali Akbar Dehkhoda defines Chelleh (Night of Forty) or Yalda as "the longest night of the year that Iranians consider it auspicious. Iranians celebrated this night with gathering around the fire and dancing at night. They set and design tables and they put hearth, scent bottle and a variety of food such as seasonal food like bread, cookies, pudding and meat on it.”
Every year, on December 21, Iranians around the world celebrate the arrival of winter, the renewal of the sun and the victory of light over darkness on Yalda Night, which is one of the most ancient Persian festivals. The festival dates back to the time when a majority of Persians were followers of Zoroastrianism prior to the advent of Islam.

Origin
Yalda, which means birth, is a Syriac word imported into the Persian language.
Early Christians linked this very ancient Persian celebration to Mithra, goddess of light, and to the birth anniversary of Prophet Jesus (PBUH).
Iranreview writes that in most ancient cultures, including Persia, the start of the solar year has been marked with the celebration of the victory of light over darkness, and the renewal of the sun.
According to Persiantribune, Early Christians used the term Yalda specifically with reference to the birth of Mithra, goddess of light.
Because Yalda is the longest and darkest night, it has come to symbolize many things in Persian poetry; separation from a loved one, loneliness and waiting. After Yalda, a transformation takes place – the waiting is over, light shines and goodness prevails.
According to Iranvisitor, among the Persian and many other ancient cultures, light, day and the sun were signs of God while night and darkness represented evil. People believed that light and darkness were in constant conflict. Longer days are the days of light and God's triumph and longer nights are signs of evil's victory. Yalda, the last night of autumn and the longest night of the year, finishes with light's victory at dawn with Mithra's birth. It is a turning point of the year and in the coming months, the days will grow longer and the nights will be shorter.
Some sources say Yalda is taken from Islamic times. In pre-Islamic Zoroastrian tradition the longest and darkest night of the year was a particularly inauspicious day, and the practices of what is now known as "Shab-e Chelleh" were originally customs intended to protect people from evil during that long night.
People were advised to stay awake most of the night, lest misfortune should befall them, and people would then 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.
In addition to Iran, Central Asian countries such as Afghanistan, Tajikistan, Uzbekistan, Turkmenistan and some Caucasian states such as Azerbaijan and Armenia share the same tradition and celebrate Yalda annually at this time of the year.
Traditions
Yalda is a time when friends and family gather together to eat and read poetry (especially Hafez) until well after midnight. Fruits and nuts are eaten and pomegranates and watermelons are particularly significant. Old texts say the red color in these fruits symbolizes the crimson hues of dawn and glow of life, invoking the splendor of Mithra.

 
Food
Food plays a central role in any form of the celebrations in Iran. In most parts of the country the extended family come together and enjoy a fine dinner.
The main dish of the night, is Fesenjan, the rich, tangy Iranian chicken stew with walnuts and pomegranate which is a highlight of the Persian fest Yalda. However, each city in Iran has its own traditions regarding the type of food to be served for dinner at Yalda night.
Gilan in northern part of Iran, serves sabzi polo, dish of rice and chopped herbs, along with whitefish or smoked fish during the night of Yalda.
But in Shiraz the special dish is havij polo, carrot rice. The dish is served with Persian roasted saffron and lemon chicken.
Anar Polo in Qazvin, Khashil in Tabriz, Khagineh Khorma in Zanjan, Kalam Polo in shiraz and Nardoon Stew in Mazandaran province are among other kinds of dishes served in different parts of Iran during the longest night of the year.

Nuts
Nuts are among foods consumed at Yalda night, in the form of sweet nuts together with dried berries and raisin. Eating nuts is said to lead to prosperity in days to come.
The red-colored fruits symbolize the crimson hues of dawn and glow of life invoking the glory of Mithra, old tales say. A wide variety of fruits and sweetmeats specifically prepared or kept for this night are served. Fruits common to the celebration include watermelon and pomegranate.

Pomegranates
Placed on top of a fruit basket, pomegranate was considered as a symbol of rebirth and breaking it brought joy to people's lives. The purple outer covering of a pomegranate symbolizes birth or dawn, and their bright red seeds the glow of life. Of course, it must be said that most of the fruits in Yalda night at that period brought blessing in their life.

Watermelon
Watermelons were put on the tables because of their spherical shape which was the symbol of the sun. Watermelon is consumed more than every other fruits. Some believe if you eat watermelon at Yalda, you won’t be hurt by the diseases and coldness of the coming winter, Dreamofiran writes. Watermelon is decorated with creative styles to add more beauty to the night.

Praying
On Yalda night, the oldest member of the family says prayers expressing gratitude to God for previous year's blessings, and prays for prosperity in the coming year, and the other members of the family raise their hands to the sky saying 'Ameen'. After paraying the oldest member cuts the watermelon which symbolizes the removal of sickness and pain from the family, and gives everyone a share.

Reading poems out loud
“The true morning will not come, until the Yalda Night is gone," the great 13th century Persian poet Sa'di writes.
One of the other traditions of Yalda night, which has been added in recent centuries, is getting a ‘Hafez reading’ from the book of great Persian poet Shamsu d-Din Muhammad Hafez-e Shirazi, the Iranian poet of 14th century AD. 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. What is expressed in that poem is believed to be the interpretation of the wish and whether and how it will come true. This is called Faal-e Hafez (Hafez Omen).

Gifts
Another custom performed in certain parts of Iran on the night of Chelleh involves young engaged couples. At Yald night, the fiance sends a basket with edible arrangement including some kinds of fruits and a variety of gifts to his fiancee. In some areas, the girl's family also do the same by returning the favor and sending gifts back for the young man.
Yalda night, for centuries served as a social occasion, was officially added to Iran's List of National Treasures during a special ceremony back in 2008.