Yalda Night was one of the holy nights in ancient Iran and included in the official calendar of the Iranian Achaemenid Empire from at least 502 BCE under Darius I. Many of its modern festivities and customs remain unchanged from this period.
Ancient peoples such as the Aryans and Indo-Europeans were well attuned to natural phenomena such as the changing of seasons, as their daily activities were dictated by the availability of sunlight, while their crops were impacted by climate and weather. They found that the shortest days are the last days of autumn and the first night of winter, and that immediately after, the days gradually become longer and the nights shorter. As such, the winter solstice, as the longest night, was called “The night of sun’s birth (Mehr)” and considered to mark the beginning of the year.