Shab e Yalda is an historical celebration of triumph of light over darkness. This is like most of Persian feasts, related to astronomy and nature. The last day of Azar (December 20th or 21st) in Persian calendar is the longest night of the year which is a symbol of evil which has no more power over light as a symbol of goodness.
Friends, family, and relatives get gather to celebrate the feast by ready poetries like Hafez, Shahnameh (The Book of King), or listening to tales which are told by elders. Eating fruits such as pomegranate, watermelon, and other available fruits has been part of inseparable dishes in this celebration.