 'Shab-e Chellah' refers to "the night opening the initial forty-day period of the three-month winter", from which the name "Chelleh", "fortieth", derives.In Persian folklore, there are three special 40-day periods.The winter periods are known as the "great Chelleh" period ( to , 40 full days), followed by the "small Chelleh" period ( to , 20 days + 20 nights = 40 days and nights).