Yalda means birth. It is a Syriac word imported into the Persian language. It is also referred to as Shab-e Chelleh, a celebration of winter solstice on December 21, which is the last night of Autumn and the longest night of the year.
This celebration goes back to when the majority of Persians were followers of Zoroastrian religion. Ancient Persians believed that evil forces were dominant on the longest night of the year and that the next day belonged to the Lord of Wisdom, Ahura Mazda.
Every Yalda night people are gathering with their loved ones, eat, drink, and read poetry (mostly from Hafez) to pass the darkest evening of the year with laughter and joy. Watermelon and pomegranate are the two main fruits served during this joyful night.