Yaldā is celebrated on the eve of the first day of the winter 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
(Persia) and continues to be celebrated to this day since 4000 years
ago. 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.