Zayeshmehr which is known as Yalda and Shab-e Cheleh in Persian is celebrated on the eve of the first day of the winter (December 21-22) in the Iranian calendar, which falls on the Winter Solstice and forty days before the next major Iranian festival "Jashn-e Sadeh (fire festival)".|
As the longest night of the year, the Eve of Zayeshmehr or the Birth of Mithra (Shab-e Yalda) is also a turning point, after which the days grow longer. It symbolised the triumph of Light and Goodness over the powers of Darkness.
Yalda celebration has great significance in the Iranian calendar. It is the eve of the birth of Mithra, the Sun God, who symbolised light, goodness and strength on earth. Shab-e Zayehmehr is a time of joy. The festival was considered pone of the most important celebrations in ancient Iran and continues to be celebrated to this day, for a period of more than 5000 years.
Yalda is a Syriac word meaning birth (NPer. milād is from the same origin) in the 3rd century CE, Mithra-worshippers adopted and used the term 'yalda' specifically with reference to the birth of Mithra.