The belief that securities market returns on Mondays are less than the other days of the week, and are often negative on average. This effect has been observed in both American and foreign exchanges. Studies have documented it since the 1920s, but no theory has adequately explained the reasons it exists. Studies have suggested the existence of a Monday effect for a diverse range of securities, from equities to debt to commodities. However, since the mid-1970s or mid-1980s (depending on the study and methodology), large firm securities seem to have exhibited what might be called a 'reverse Monday effect,' in which differences between Monday trading and the rest of the week are not statistically significant. Small firm securities have continued to exhibit the Monday effect. It is also known as the "weekend effect."
The tendency of stocks to produce lower-than-average returns on Mondays compared to other days of the week. One study indicated most of the poor performance during Mondays occurs during the first hour of trading.