Yaldā also known as Shab-e Cheleh, is celebrated on the eve of the first day of the winter, December 21 [first of Dey] in the Iranian calendar, which falls on the Winter Solstice. It celebrates the birth of Sun god Mithra.The festival was considered extremely important in pre-Islamic Iran and continues to be celebrated to this day, a period of more than 6000 years. Some historians believe that the festival spread to Europe through contacts between the Roman and Persian empires and was eventually replaced by Christmas;
a theory that accounts for the celebration of Christmas on 25 December, rather than the later date of January 6 that is believed to be the correct date of birth of Christ by eastern orthodox church …… The Iranian Jews … in addition to “Shab e Cheleh” also celebrate the festival of “Illanout”, tree festival, at around the same time…. read more
Update: Thanks to Dear Professor here is a correction of the original text from wikipedia:
The fact that Eastern/Orthodox Christians use Jan 6 is only due to a “technical” fault in the Julian/Roman calendar they still use … not on their using the “birth date of Jesus”!I guess Persian and Chaldean astronomers were more advanced than their Roman and Medieval Christian colleagues.