It has been a punishing October for this aging bull market. And the degree to which stock-market sentiment has turned sour may be best illustrated by an unwelcome record.

If the S&P 500 index SPX, +1.59% ends lower in either of the final two sessions of October, it will mark its 17th down day, representing that the highest number of down days in a month since April 1970, according to Dow Jones Market Data. The broad-market index’s current number of declining sessions already ranks as the most since 2008.