The festival was considered extremely important in pre-Islamic Iran and continues to be celebrated to this day, for 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.