Yalda is celebrated on the longest night of the year, December 21st. it is supposed to mark birth of Mithra. This is perhaps the only ancient Iranian ceremony except Nowruz that has retained its importance among Iranians. Light, day and sunshine are assumed to be the signs of Ahuramazda, whereas night, darkness and cold are thought to be associated with evil Ahriman. Because the days get longer after the longest night of winter and the warmth and light of the sun increase, the night is supposed to be the time for the rebirth of sun (Mehr). The obligatory trappings of the Yalda are an enormous variety of nuts and fruits, particularly watermelons.