A calendar effect is any actual or hypothesized stock market trend based on the calendar, such as rises and falls associated with particular days of the week or months of the year.Examples include:Halloween indicator (or the 'Sell in May' principle)January effectMark Twain effectMonday effectWeekend effect
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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.