New York CNN  —  US stocks are about to achieve a feat so rare that it’s only ever happened a handful of times. The S&P 500 is set to gain over 20% this year after rising 24% in 2023. Back-to-back gains of over 20% would be the best performance for the benchmark index since 1997 and 1998, according to data from FactSet. It’s an extraordinary event for the modern-day version of the index. (Precursors also racked up that kind of performance three other times, in 1927 and 1928, 1935 and 1936 and