Yalda night,The longest night in year.

The 13th century Iranian poet Sa'di wrote in his Bustan: "The true morning will not come, until the Yalda Night is gone". Following the Iranian calender reform of 1925, which pegged some seasonal events to specific days of the calendar, Yalda came to be celebrated on the night before and including the first day of the tenth month (Dey). Subject to seasonal drift, this day may sometimes fall a day before or a day after the actual Winter Solstice.