 Iranians (Parthians, Persians, and others) were exposed to their religious observances including Yalda, which in Syriac means "birth" and was the Syriac Christian name for Christmas.It was celebrated on the eve of the winter solstice because that date fell nine months after the Annunciation.Although it is not clear when and where the Syriac term passed into Persian, gradually 'Shab-e Yalda' and 'Shab-e Chelleh' became synonymous.