The 7.19% decline the S&P 500 has seen over the first five trading days of August is the worst start to the month in the S&P 500's history going back to 1928. The next worst start to August came in 1990 when the S&P declined 5.99% over the first five trading days. For what it's worth, the index continued to decline for the remainder of August in 1990 by 3.66%. It then rallied slightly from September through the end of the year.
There have been three other starts to August where the S&P declined more than 3% (1982, 2002, 2004), and in each of these instances, the index gained significantly for the remainder of August. In 1982, the S&P then went on to gain 35.61% for the remainder of the year, and in 2004, the index gained 13.91% over the rest of the year. In 2002, the index was essentially flat for the remainder of the year after dropping 3.82% in the first five days of August.