Iranians celebrate YALDAA: the the night of the winter solstice (around Dec 21). YALDAA is a tradition older than Islam that is celebrated by all Iranians (regardless of religion).
It is characterized by family reunions and feasting with grandparents and friends, the telling of stories and the reading of poetry to the younger generations.
It is also virtually obligatory to eat pomegranate and summer fruits like water melon and local sweet melons, along with dried fruits and nuts, in the belief that they will keep illness at bay until the spring.
In ancients times, Iranians believed that MAZDA (God) created "the day", a time of production and work, and that AHRIMAN (evil) created "the night', a time of dark secrets and wild attacking animals. The people believed that during the longest night of the year, YALDAA, the greatest war between the good and evil forces was fought. The following day, when light shone over the dark night, they knew that the days now would grow longer and they would enjoy more of the sun.
In the old Persian mythology, the God of love, friendship, and light, the God of Sun, Mitra (Mithra, Mehr) was born in the longest night of the year . This night is called YALDAA which means the Birthday.
The major Persian feasts are focused, like YALDAA, on the changing of the seasons. The biggest is the Iranian New Year, which occurs with the spring equinox, around March 21.