Calendar effect

A calendar effect (or calendar anomaly) is any market anomaly, different behaviour of stock markets, or economic effect which appears to be related to the calendar, such as the day of the week, time of the month, time of the year, time within the U.S. presidential cycle, decade within the century, etc...

Some people believe that if they do exist, it is possible to use market timing.

Seasonal patterns are not confined to prices; many other systems can exhibit the same kind of calendar effect. However, the term is most often used in an economic context.