Yalda is the longest night of the year. Ancient Iranians believed that at the end of this longest night, which they believed was evil, darkness was defeated by light (Sun) allowing the days to become longer. 

This celebration comes at the beginning of the Iranian month of Day, which was also the name of the pre-Zoroastrian Iranian creator god (deity). Later he became known as the god of light, from where the English word “day” has its roots. 

The birth of the sun and beginning of the winter has become the beginning of the year and source of celebration in many cultures and traditions. Early Christians related this very ancient Persian celebration to Mithra, god of light, and linked it to Christ's birthday. Today, the date for Christmas is slightly off from Yalda, but they are celebrated in many similar ways, staying up all night, singing and dancing, gatherings with family and friends, lighting candles, and eating special foods.

Coinciding with the very beginning of winter, Yalda is also an occasion to celebrate the end of the harvest of the previous year and to wish and pray for the prosperity of the next year’s harvest.

It was said that Mithra was born out of the light that came from within the Alborz mountains. Ancient Iranians would gather in caves along the mountain range throughout the night to witness this miracle together at dawn. They were known as 'Yar-e Ghar' (Cave Mates). 

In Iran today, despite of the advent of Islam and Muslim rituals, Shab-e Yalda is still celebrated widely. It is a time when friends and family gather together to eat, drink and read poetry (especially Hafiz) 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, invoking the splendour of Mithra.