This night is called Yalda which means rebirth (of the sun), and it was celebrated for the triumph of light over darkness. They built fires on sundown of the last day in fall and kept them burning until the first rays of the sun the following day.
Shab-e Yalda (“Yalda night” Persian: شب يلدا) or Shab-e Chelleh (“night of forty”, Persian: شب چله ) is an Iranian festival celebrated on the “longest and darkest night of the year,” also is a turning point, after which the days grow longer. Yalda is a winter solstice celebration, that is, in the night of the Northern Hemisphere’s winter solstice. Calendrically, this corresponds to the night of December 20/21 (±1) in the Gregorian calendar, and to the night between the last day of the ninth month (Azar) and the first day of the tenth month (Dey) of the Iranian civil calendar.
The longest and darkest night of the year is a time when friends and family gather together to eat, drink and read poetry (especially Hafez) until well after midnight. Fruits
and nuts are eaten and pomegranates and watermelons are particularly significant. The red colour in these fruits symbolizes the crimson hues of dawn and glow of life. The poems of Divan-e Hafez, which can be found in the bookcases of most Iranian families, are read or recited on various occasions such as this festival and Nowruz. Shab-e Yalda was officially added to Iran’s List of National Treasures in a special ceremony in 2008.
‘ The sight of you each morning is a New Year Any night of your departure is the eve of Yalda’ (Sa’adi)
‘With all my pains, there is still the hope of recovery As the eve of Yalda, there will finally be an end’ (Sa’adi)
Source: Wikipedia
