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There are all together three 40-day periods, one in summer, and two in winter. The two winter periods are known as the “great Chelleh” period (1 Day to 11 Bahman, 40 full days), followed/overlapped by the “small Chelleh” period (10 Bahman to 30 Bahman, 20 days + 20 nights = 40 nights and days). Shab-e Chelleh is the night opening the “big Chelleh” period, that is the night between the last day of autumn and the first day of winter.
