Turn-of-Month Effect
The Turn-of-Month Effect is a historically observed pattern in which US equity markets have tended to generate a disproportionate share of their monthly returns in a narrow window surrounding the final trading day of one month and the first few trading days of the following month.
Research by Ariel (1987) and later by Lakonishok and Smidt (1988) documented that a significant portion of the historical equity market return accrued in a cluster of days around the calendar month-end. In some studies, the turn-of-month window spanning roughly the last trading day through the fourth trading day of the new month accounted for the entirety of average monthly equity returns, with the rest of the month generating near-zero average returns.
Several structural explanations have been proposed. Payroll processing cycles — in which companies pay employees at or near month-end — generate predictable flows into savings and retirement accounts at the beginning of each month, with a portion directed toward equity investments. Institutional investors with month-end performance evaluation cycles may deploy capital early in the new month. Month-end rebalancing flows for balanced and multi-asset portfolios, which sell fixed income and buy equities when equities have underperformed to restore target allocations, also cluster near the end of the month.
The turn-of-month pattern has been documented in multiple international equity markets beyond the US, suggesting that it is not purely a US institutional artifact but may reflect a more universal feature of how money flows through financial systems on a monthly calendar cycle.
As with most well-publicized calendar anomalies, the pattern has likely become less exploitable over time as awareness of it has spread. Institutions anticipating month-end buying may position ahead of it, pulling the anticipated return into earlier in the month or the prior month-end window. The reliability of the pattern in any specific month depends on the broader market environment, liquidity conditions, and whether there are countervailing macroeconomic events.
The turn-of-month effect is a historical observation useful for contextualizing short-term price behavior rather than a reliable foundation for trading strategies.