On the eve of the first day of winter, the winter solstice, sometime between December 21 and 22, a Persian nocturnal celebration will be celebrated in Iran. It is commonly known as 'Yalda' (Night of Birth), or 'Shab-e Chelle', for 'chelle' meaning 40, and Yalda celebration takes place 40 days prior to the Zoroastrian fire festival of Sadeh, according to the Circle of Ancient Iranian Studies.