(This afternoon, the index is up 0.2% from Friday's close.)
The last time that happened was in November 2004.
If the S&P 500 closes up again Tuesday, the win streak will extend to nine days (which also happened in November 2004). The S&P 500 has had 40 such nine-day win streaks since 1928.
One has to go all
The longest streak ever was in April 1971, when the S&P 500 went 14 days in a row without sinking. That is also the only occurrence ever where the S&P 500 has closed up more than 12 days in a row.
According to Bespoke Investment Group, the average S&P 500 return on the ninth day of a streak like this is slightly negative. However, the median return is positive:
...the average return of the S&P 500 the day after eight straight days of gains is a decline of 0.13%. While the average return is negative, the index has actually been positive on day nine (20 times) more often than it has been down (15 times). So more often than not, the S&P has been positive on day nine than it has been negative. Overall, the longest winning streak for the S&P 500 over the last fifty years has been 14 trading days in April 1971.
The histogram below shows the frequency of win streaks over seven days in a row. The length of the win streak is plotted along the x-axis and the number of streaks is displayed on the y-axis.