Yalda Night is one of the great celebrations of ancient Persia that is still celebrated. This night refers at sunset in Persian calendar (date) on Azar 30th, last day of autumn (21st of December) until sunrise on Dey 1st, first day of winter (22nd of December). Iranians and many other nations celebrate it. This night is also very important from a scientific point. Because it is the longest night of the year in the northern hemisphere of the earth.