The table and chart below show the cumulative gain of the S&P 500 during the current bull market by weekday. In other words, for each day we show how an investor would have done had they only been invested in the market on that specific day. We also show the S&P 500's average daily change, as well as the percentage of time that the S&P 500 trades higher on that day.
As shown, although today is an exception, Thursday's have been the best day of the week for the current bull market. So far this bull market, Thursday's have been positive 58% of the time, averaging a daily gain of 17 basis points. This is nearly twice the average daily gain for all days during this bull market. As a result, had you only been invested in the S&P 500 on Thursdays so far in this bull market, your cumulative gain would have been +32.9%.
While Thursdays are strong, Fridays have been weak during the entirety of this bull market, although more recently in 2012 Fridays have been stronger (more on that later). Since 3/9/09, if you had been invested in the S&P 500 just on Fridays, your cumulative gain would actually be a loss of 0.6%. The phrase Thank Goodness It's Friday may be true in most cases, but not during this bull market.