The Monday Myth: Best Day to Buy Index Funds (Updated)
Refresh notice. This is an updated version of the May 2018 Monday Myth post, extended with eight additional years of market data through mid-2026. The original analysis and comments remain part of the same conversation.
Investing Analysis · Refresh
The Monday Myth: Best Day to Buy Index Funds (Updated)
Eight more years of data. A pandemic, a bear market, an AI rally. Here's whether the "best day to buy" question got any more useful in 2026.
"Appreciate the analysis. I'm getting my MBA at the moment and was planning to propose this strategy and do the related regression analysis. You saved me the time of going down this rabbit hole, so now I need to pivot and see if I can find a better strategy."
John A, comment on the original post, December 2020
That comment stuck with me. Not because it was flattering, though it was, but because the reader saying it was exactly the type of person the original post was written for. Technical, statistically literate, ready to run the regression, willing to update on the result. If someone with that mindset is asking whether Monday is the best day to buy index funds, the question isn't as settled as most personal-finance advice makes it sound.
Eight years of new data have piled up since I first ran that analysis. A pandemic, a bear market in stocks and bonds together, an AI-driven rally, and two Fed cycles. If the day of the week ever mattered for index fund investors, this stretch would have exposed it. Here's the honest update.
The one-line answer up front: no, there's still no day that consistently beats the others. The original conclusion holds, and the 2020-2026 stretch actually makes it stronger.
Every year of new data makes the case weaker for timing your DCA to a specific weekday, not stronger.
The Original
What the 2018 Analysis Actually Found
The 2018 post used VFINX (Vanguard S&P 500 Index Fund Investor Shares) daily closing prices from January 3, 2000 through April 27, 2018. Simulating a $1,000 weekly investment on each specific day of the week showed differences of less than 1 percent across weekdays after 18 years of compounding. Monday narrowly edged out the others, but the gap sat inside statistical noise.
The mechanical finding worth remembering was that Monday hosted the week's lowest price about 33 percent of the time, above the roughly 20 percent you'd expect from random distribution. But the practical impact on total returns was still negligible. A follow-up simulation showed random day selection actually outperformed Monday-only selection in most trials.
The takeaway wasn't "Monday is best." It was "the day doesn't matter enough to build a strategy around."
The Stress Test
What Changed From 2018 to 2026
Eight years is a long window, and this one delivered multiple stress tests for any day-of-week hypothesis.
February 2018 brought the volatility spike known as Volmageddon, when the short-volatility trade unwound over a single week. Q4 2018 saw a sharp correction driven by the Powell pivot on rate policy. The COVID crash of February and March 2020 was the fastest bear market in history, followed by the fastest recovery. 2022 was one of the worst years for both stocks and bonds in decades. 2023 and 2024 delivered the AI rally that pulled the S&P higher on the back of a handful of tech names. 2025 and 2026 have added their own noise, including the SpaceX IPO in June 2026, the largest listing ever.
If a Monday-buys-cheaper effect were a real, exploitable pattern, this stretch would have amplified it. Concentrated volatility events tend to reveal any real day-of-week bias because daily moves get larger and any pattern compounds faster.
The Refreshed Verdict
Best Day to Buy Index Funds in 2026: Still None
Re-running the simulation on VFIAX (Vanguard's Admiral share class of the same S&P 500 index fund) from January 3, 2000 through July 20, 2026 gives 6,455 trading days and 1,340 weekly investing decisions to work with. The specific numbers shift, but the direction is knowable in advance from decades of academic research on the Monday effect: it has weakened over time, not strengthened. The 2026 numbers confirm exactly that.
$1,000 Weekly Investment, Jan 2000 - Jul 2026
| Weekday | Invested | Ending Value | Return |
|---|---|---|---|
| Monday | $1,211,000 | $5,333,718 | 340.44% |
| Tuesday | $1,326,000 | $5,819,221 | 338.86% |
| Wednesday | $1,325,000 | $5,822,556 | 339.44% |
| Thursday | $1,299,000 | $5,703,270 | 339.05% |
| Friday | $1,294,000 | $5,685,222 | 339.35% |
Monday edges out at 340.44 percent. Tuesday brings up the rear at 338.86 percent. The spread between best and worst weekday, across 26 years of compounding: 1.58 percentage points. That's inside noise for any practical investment decision.
The invested totals differ across weekdays because most US market holidays fall on Mondays (MLK Day, Presidents Day, Memorial Day, Labor Day), which cost the Monday investor 115 fewer $1,000 buys than the Tuesday investor over 26 years. Once you normalize to return percentage, the picture flattens completely.
The academic story on the Monday effect has been consistent for a long time. Original documentation in US markets goes back to the 1970s, when researchers found small negative Monday returns for stocks. Follow-up work through the 1990s and 2000s has shown the effect diminishing, disappearing, or reversing in most developed markets. The likely explanation is exactly what one of the original commenters, Enoch, gave in 2018: once an inefficiency becomes public knowledge, arbitrageurs trade it away. Famous inefficiencies don't survive.
The 2020s specifically make the point sharper. COVID's daily volatility was so extreme that any day-of-week signal was completely swamped by the size of individual moves. You cannot pick a systematically better weekday in an environment where a random Tuesday can produce a 9 percent drop.
Which Weekday Hosted the Week's Low Close (2000-2026)
| Weekday | Weeks | Share |
|---|---|---|
| Monday | 393 | 29.33% |
| Tuesday | 254 | 18.96% |
| Wednesday | 185 | 13.81% |
| Thursday | 196 | 14.63% |
| Friday | 312 | 23.28% |
1,340 weeks in the sample. Monday still hosts the weekly low more often than any other day, but at 29.33 percent it's down from the roughly 33 percent figure in the 2000-2018 window. The effect is still there. It's just fading exactly the way the academic literature has predicted for decades.
Two other things worth noticing in that table. Friday is the second-most-common weekly low, not something the naive "buy Monday, sell Friday" story would predict. And Wednesday hosts the low the least often of any weekday, at 13.81 percent, yet the Wednesday DCA return of 339.44 percent finished second best. Any strategy based on "buy on the day of the weekly low" would have missed most weekly lows and would still have produced returns statistically indistinguishable from just picking any random weekday and sticking with it.
The Meta
Why This Question Keeps Coming Back
The MBA comment I opened with tells you something about who keeps searching for this. It's not typically dividend investors looking for tactical edge. It's technical people, statistically literate, who see a plausible pattern and want to test it before committing to a strategy. That's a good instinct, and it's exactly the mindset that produced the original Monday effect papers in the first place.
The problem isn't the question. It's the size of the effect versus the size of the friction to exploit it. Even in the 1970s and 1980s, when the Monday effect was documented, the excess return was fractions of a percent per year. In a taxable account with any transaction cost or timing friction at all, that gets consumed by execution.
The better version of the question isn't "which day is best" but "how much variance in my long-run return is attributable to timing decisions I could realistically make?" The honest answer for a weekly-DCA index fund investor is: less than one percent. Something else in your investing life is worth more attention.
The best day to buy index funds is whichever day your automatic transfer already runs. If you're setting one up for the first time, pick a day and stop thinking about it. Years in the market swamp any effect you could have squeezed out of a specific weekday.
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