In the most ancient civilization, Eranshahr (Iranshahr, Iran or well known as Persia), the start of the solar year has been marked to celebrate the victory of light over darkness and the renewal of the Sun or known as The Eve of the Birth of Mithrâ, the Sun God

'Shab-e Chelle', celebrated on 21 December, has great significance in the Iranian (Zoroastrian) calendar. It is the eve of the birth of Mithrâ, the Sun God, who symbolized light, goodness and strength on earth. This night is a time of joy. The Cult of the Sun was first introduced to Iran thousands of years ago by migrant Aryans. Mithrâ, the Sun God remained a potent symbol of worship throughout the following centuries. Over these 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 Mithrâ was transferred to 25 December.

The last day of the Zoroastrian month ‘Azar’ is the longest night of the year, when the forces of Ahriman, the power of ignorance, are assumed to be at the peak of their strength. While the next day, the first day of the month ‘Day’ known as "khoram rooz" or "khore rooz" (the day of sun) belongs to Zoroastrian God, Ahura Mazda, the Supreme existence of Wisdom. Since the days are getting longer and the nights shorter, this day marks the victory of Sun over the darkness. The occasion was celebrated in the Jashn (festival) of "Daygan" dedicated to Ahura Mazda, on the first day of the month "Day"

In the Atashgah (Fire Temples), Fires would be burnt all night to ensure the defeat of the forces of Ahriman. There would be acts of charity and a number of deities were honored and prayers performed to ensure the total victory of sun that was essential for the protection of winter crops. There would be prayers to Mithra (Mehr), the Izad (god) of promise & faith, and feasts in his honor, since Mithrâ is the responsible for protecting "the light of the early morning" known as "Havangah" It was also assumed that Ahura Mazda would grant people's wishes, specially those with no offspring had the hope to be blessed with children if performed all rites on this occasion.

The Persian tradition merged in ancient Rome, in a festival to the ancient god of seedtime, Saturn. The Romans exchanged gifts, partied and decorated their homes with greenery. Following the Persian tradition, the usual order of the year was suspended. Grudges and quarrels forgotten, wars would be interrupted or postponed. Businesses, courts and schools were closed. Rich and poor became equal, and children headed the family. Candles and lamps chased away the spirits of darkness.

With the spread of Christianity, Christmas celebration became the most imposed Christian festival. In the third century various dates, from December to April, were celebrated by Christians as Christmas. January 6 was the most favored day because it was thought to be Jesus' Baptismal day (in the Greek Orthodox Church this continues to be the day to celebrate Christmas), but as there was no documented evidence of Jesus’s birthdate, finally in year 350 ac, by referring directly to the birthday of Mithra , the 25the day of the tenth month, December 25 (deca = ten)  was adopted in Rome and gradually almost the entire Christian Church agreed to that date, which coincided, with Winter Solstice and the festivals, Sol Invicta and Saturnalia. Many of the rituals and traditions of the pagan festivals were incorporated into the Christmas celebration and are still observed today without making references to its origin.

It is not clear when and how the world "Yalda" entered the Persian language. The massive persecution of the early Christians in Rome brought many Christian refugees into the Sassanid Empire and it is very likely that these Romans introduced and popularized "Yalda" in Iran. Gradually "Shab e Yalda" and "Shab e Cheleh" became synonymous and the two are used interchangeably.

Today "Shab e Cheleh" is a very important occasion for Iranians, when friends and family gather together to eat, drink and read poetry until well after midnight. A green & live tree (mostly a cypress, later a pine tree, because these trees stay green all year trough) symbolized the immortality and fruits and nuts are eaten and pomegranates and watermelons are particularly significant. The red color in these fruits symbolizes the crimson hues of dawn and glow of life, invoking the splendour of Mithra.

'With all my pains, there is still the hope of recovery

Like the eve of Yalda, there will finally be an end' (Sa'adi)