Day-of-week Effects

What are Day-of-week Effects?

Day-of-week effects are calendar tendencies in which average return, volatility, or volume differs by weekday. The classic examples come from the equity literature: the Monday (or weekend) effect, where Mondays showed the weakest average returns in twentieth-century US stock data, and 'turnaround Tuesday', the folk tendency for Tuesday to reverse Monday weakness. Measurement is simple: bucket daily returns by weekday and compare each bucket's average return, win rate, and range, the same machinery as month-of-year seasonality at a finer grain.

Two caveats define the concept. The measured effects are small relative to daily noise, so they only appear in aggregates, and several classic anomalies weakened or vanished in the decades after publication. Weekday statistics are context to re-verify on your own market and sample, often alongside intraday time-of-day effects, not standing edges.

How traders use it

• Profiling a market's week: bucketing range and volume by weekday shows where movement historically concentrates, which helps schedule attention and avoid forcing trades into typically quiet sessions.
• Filtering a strategy: testing whether a system's results concentrate on particular weekdays before adding a day filter, with out-of-sample checks so noise is not mistaken for pattern.
• Planning around recurring flows: weekly closes, options expiries, and Monday gaps give some weekdays a distinct character worth preparing for even when average returns are statistically indistinguishable.