The weekend effect is a phenomenon in financial markets in which stock returns on Mondays are often significantly lower than those of the immediately preceding Friday.

(The weekend effect is sometimes known as the Monday effect, although that theory states that returns on the stock market on Mondays will follow the prevailing trend from the previous Friday. If the market was up on Friday, it should continue through the weekend and, come Monday, resume its rise, and vice versa.

Some theories that attempt to explain the weekend effect point to the tendency of companies to release bad news on a Friday after the markets close, which then depresses stock prices on Monday.