The last night of autumn in Persian heritage.

The word Yalda derives from a Syriac term signifying "birth." Due to it being the longest night, "Yalda" was celebrated as the day Mithra was born. The calibration carried on in to Zoroastrianism, ancient Greece, Roman Empire and was adopted by Christians as the birthday of Christ. It is an ancient Indo-European calibration, and a part of the festival was to decorate evergreen trees, which symbolize life and strength, even in today's Iran. A Winter Solstice festival - Shabe Chelle - had already been celebrated throughout the Iranian world even prior to the introduction of the word Yalda during the early Sassanid Era.
when is it?
when is Yalda?
when was the Christ born?