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Calendar Effect is any economic effect, particularly in stock markets, which appears to be related to the calendar. It is the tendency of stocks to perform differently at different times, including such anomalies as the January effect, month-of-the-year effect, day-of-the-week effect, and holiday effect. Market prices are often subject to seasonal tendencies because the availability and demand for an item is not constant throughout the year. For example, natural gas prices often rise in the winter because that commodity is in demand as a heating fuel. In the summer, when the demand for heat is lower, prices typically fall.