HAPPY YALDA !

Yalda , Shab-e Yalda "Night of Birth", or Zayeshmehr "Birth of Mithra", or Shab-e Chelleh "Night of Forty" is the Persian winter solstice celebration which has been popular since ancient times. Yalda is celebrated on the Northern Hemisphere's longest night of the year, that is, on the eve of the Winter Solstice. Depending on the shift of the calendar, Yalda is celebrated on or around December 20 or 21 each year. 
Yalda has a history as long as the religion of Mithraism. The Mithraists believed that this night is the night of the birth of Mithra, Persian angel of light and truth. At the morning of the longest night of the year the Mithra was born.
http://en.wikipedia.org/wiki/Yaldā