Yalda is a Syriac word meaning birth. Mithra-worshippers used the term 'yalda' specifically with reference to the birth of Mithra. As the longest night of the year, the Eve of Yalda (Shab-e Yalda) is also a turning point, after which the days grow longer. In ancient times it symbolised the triumph of the Sun God over the powers of darkness. The Cult of the Sun was first introduced to Iran thousands of years ago by migrant Aryans. Mithra, the Sun God remained a potent symbol of worship throughout the following centuries. Centuries later, during the Achaemenid era, Mithra became a principal deity, equal in rank to Ahura Mazda (the god of all goodness) and Anahita (goddess of water and fertility).
In Sasanian times, Zoroastrianism became Iran's official religion, but Mithra's importance remained undiminished. This is evident from the bas-reliefs as Naqsh-e Rustam and Tagh-e Bustan. At Naqsh-e Rustam, Anahita bestows the royal diadem upon Nasri, the Sasanian King. At the investiture of Ardeshir I, Ahura Mazda bestows this diadem to the new King. At Tagh-e Bustan too, Ahura Mazda is again conferring the royal diadem upon Ardeshir II. Mithra is always present as a witness to these ceremonies. Over the centuries Mithraism spread to Greece and Ancient Rome via Asia Minor, gaining popularity within the ranks of the Roman army. In the 4th century AD as a result of errors made in calculating leap years and dates, the birthday of Mithra was transferred to 25 December. Until then Christ's birthday had been celebrated on 6 January by all branches of the Christian Church. But with the cult of Mithra still popular in Roman Europe, the Christian Church adopted many of the Mithraic rituals and proclaimed 25 December as the official birthday of Christ. Today the Armenian and Eastern Orthodox Churches continue to celebrate 6 January as Christ's birthday.
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.
Because Shab-e Yalda is the longest and darkest night, it has come to symbolise many things in Persian poetry; separation from a loved one, loneliness and waiting. After Shab-e Yalda a transformation takes place - the waiting is over, light shines and goodness prevails.
' The sight of you each morning is a New Year
Any night of your departure is the eve of Yalda' (Sa'adi)
'With all my pains, there is still the hope of recovery
Like the eve of Yalda, there will finally be an end' (Sa'adi)