We looked into this and found that while the S&P 500 has not had a string of nine straight monthly gains in the last twenty years, it has happened before. Since 1930, there have actually been nine periods of at least eight straight monthly gains, and in five of those periods the S&P 500 rose for a ninth month. The longest streak occurred from 1935 to 1936, when the S&P posted gains for 12 months in a row, while the most recent streak ended in June 1996.
The table below lists each period of at least eight months along with the market's return during the ninth month. As the table details, even though 8 straight months may mean the market is "due" for a decline in the ninth month, most of the time the S&P 500 rises, with an average gain of 0.8%.