Is it better to invest in the stock market on a certain day of the week?

Do companies deliver bad news evenly spread out across every weekday? If it was clustered around a particular weekday, I could get out the day before and reinvest the day after…

I analyzed the S&P 500 from the 1920s to the 2010s to find out if my hypothesis had legs. Here’s the average of how much the S&P500 changed compared to the day before for every weekday.

As you can see, Monday’s were horrible from the 1920s to the 1980s. Even if you take out the 1920s (Great Depression), you’d still average losing 0.16% of your portfolio’s worth on any given Monday! The right strategy would have been to pull your money out on Friday (the market was also open on Saturdays until 1952) and reinvest your money on Tuesdays.

If you just invested in the S&P500 from 1960–1969, you would’ve gotten a 54% return on your investment. If you skipped Mondays, your return would’ve been 238%!

Unfortunately, it looks like that exploitable pattern closed up in the 1980s. Back to the drawing board :)