Yalda (Persian: یلدا)), Shab-e Yalda (Persian: شب یلدا), "Night of Birth", or Zayeshmehr (Persian: زایش مهر) "Birth of Mithra", or Shab-e Chelleh (Persian: شب چلّه, Azerbaijani: چیلله گئجهسی; lit. "Night of Forty") is the Persian winter solstice celebration which has been popular since ancient times. Yalda is celebrated on the Northern Hemisphere's longest night of the year, that is, on the eve of the Winter Solstice. Depending on the shift of the calendar, Yalda is celebrated on or around December 20 or 21 each year.
Yalda has a history as long as the religion of Mithraism. The Mithraists believed that this night is the night of the birth of Mithra, Persian angel of light and truth. At the morning of the longest night of the year the Mithra was born.
Following the fall of the Sassanid Empire and the subsequent rise of Islam in Persia/Iran, the religious significance of the event was lost, and like other Zoroastrian festivals, Yalda became a social occasion when family and close friends would get together. Nonetheless, the obligatory serving of fresh fruit during mid-winter is reminiscent of the ancient customs of invoking the divinities to request protection of the winter crop.
The 13th century Persian poet Sa'di wrote in his Bustan: "The true morning will not come, until the Yalda Night is gone".
Following the Persian calendar reform of 1925, which pegged some seasonal events to specific days of the calendar, Yalda came to be celebrated on the night before and including the first day of the tenth month (Day). Subject to seasonal drift, this day may sometimes fall a day before or a day after the actual Winter Solstice.
Yalda Night has been officially added to Iran's List of National Treasures in a special ceremony in 2008.
Iranian Azerbaijanis call it Chilla Gejasi, which means the beginning of the first 40 days of winter.