 Today, the two terms are used interchangeably.In Zoroastrian tradition the longest and darkest night of the year was a particularly inauspicious day, and the practices of what is now known as "Shab-e Chelleh/Yalda" were originally customs intended to protect people from evil (see "dew"s) during that long night, at which time the evil forces of Ahriman were imagined to be at their peak.People were advised to stay awake most of the night, lest misfortune should befall them, and people would then gather in the safety of groups of friends and relatives, share the last remaining fruits from the summer, and find ways to pass the long night together in good company.