The meaning of  "Yalda"

"The word Yalda derives from a Syriac term signifying "birth." A Winter Solstice festival, Jashne Shabe Cheleh 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 Iranian festivals tied to the solar calendar, Jashne Shabe Cheleh 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 European festival is also originally a mid-winter event."  Source: Encyclopedia

Jashne Shabe Cheleh
Ancient Iranian Celebration for more than 5000 years
The longest night of the year
The beginning of Winter (solstice):
  Sunday, December 21, 2008 - 04:04 AM PST