
Shabe Yaldā (Persian: یلدا) or Shabe Chelle (Persian: شب چله) is an Iranian festival originally celebrated on the Northern Hemisphere's longest night of the year, that is, on the eve of the Winter Solstice.
Following the Iranian calendar reform of 1925, which pegged some seasonal events to specific days of the calendar, Yalda came to be celebrated on the 1st day of the 10th month (Dey). Subject to seasonal drift, this day may fall a day before or a day after the actual Winter Solstice.
The word Yalda derives from a Syriac term signifying "birth." A Winter Solstice festival - Shab-e Chelle - had already been celebrated throughout the Iranian world even prior to the introduction of the word Yalda during the early Sassanid Era. Like all other festivals tied to the solar calendar, Shabe Chelle has its roots in the close contacts between Chaldea/Babylonia and Iran during the late Achaemenid period.
The process by which the term Yalda entered the Persian language is not conclusively established, but is probably attributable to Syriac Christians who received protection from the Sassanid monarchs. While these Christians that brought Yalda to Iran presumably associated the festival with the birth of Christ, the proximity of December 25th to the day of the Winter Solstice (December 21st or 22nd) eventually led to the two festivals being conflated and celebrated as one.
Contrary to some expectations, Yalda is neither etymologically nor culturally related to Yule or Yuletide, even though that festival is also originally a mid-winter event. Nothwithstanding that, as in Iran also, evergreen trees everywhere symbolize the continuity of nature, Yule has its origins in Saxon/Norse culture and was originally a lunar calendar event.
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