The Monday effect is the old idea that stocks tend to fall on Mondays. We counted how often SPY, QQQ and IWM closed higher than the day before, for each weekday, over 885 trading days from March 2023 to October 2026.
Monday did best. SPY rose on 66.5% of Mondays. QQQ and IWM also did best on Mondays.
A Monday's change is measured from Friday's close, so it includes the weekend. SPY gained 0.22% on the average Monday.
Friday was second on SPY and QQQ. Tuesday was SPY's weakest day, at 51.4%.
What to do with it
If you're thinking about buying on a certain day of the week, replay a few Mondays and Fridays from earlier weeks in Market Replay and see what your position would have done. Or write the rule as a strategy and run it in the backtester on any stock you trade.
An example day
SPY on September 21, 2026 was a Monday. It closed at $773.52, up from the Friday before.
Play it from the open in Market Replay and see what you'd have done.
| Symbol | Monday | Tuesday | Wednesday | Thursday | Friday |
|---|---|---|---|---|---|
| SPY | 66.5% | 51.4% | 56.0% | 52.8% | 56.5% |
| QQQ | 66.5% | 52.5% | 58.2% | 49.4% | 58.8% |
| IWM | 62.9% | 47.0% | 47.8% | 54.0% | 53.1% |
How we counted
Daily closes for SPY, QQQ and IWM from March 2023 to October 2026, built from TraderPier's own trade data. The close is the last regular-hours trade of the day, so it can differ a little from the official closing price. A day counts as up when its close is above the previous trading day's close, so a Monday is measured from Friday's close and the day after a holiday from the last day the market was open. A day that closed exactly flat doesn't count as up, and the first day in the data has no day before it. A half day counts as a trading day. Prices are as traded, not adjusted for dividends.
Questions
What is the best day of the week to buy stocks?
In our data, Monday. SPY rose on 66.5% of Mondays, more than on any other weekday. QQQ and IWM also did best on Monday. Friday was second on SPY and QQQ.
Do stocks go down on Mondays?
Not in our data. SPY rose on 66.5% of Mondays, and QQQ and IWM rose on most of theirs.
Do stocks go up on Fridays?
On SPY and QQQ, Friday was the second best day after Monday. SPY rose on 56.5% of Fridays.
What is the worst day of the week for stocks?
Tuesday for SPY and IWM, and Thursday for QQQ. SPY rose on 51.4% of Tuesdays.
Does this mean Monday will keep leading?
No. Our data runs from March 2023 to October 2026, and the Monday effect was first described decades ago on much longer histories. This page only tests the years we have.