Traders work on the trading floor at the New York Stock Exchange (NYSE) in Manhattan, New York City, U.S., May 20, 2022. Andrew Kelly | ReutersThe S&P 500 has done something it hasn’t in at least two decades, according to Susquehanna.Chris Murphy, Susquehanna’s co-head of derivatives strategy, noted that 97% of S&P 500 stocks are up for the third time in the past five trading days. It marks the first time since at least 2000 that the broader market index has seen such strength at the stock level.Investors watch the 97% marker because the index typically pulls back after a day with that percentage of its components up, Murphy said.He also noted that prior to last week, this week marked the first time since Aug. 25, 2015 that 97% of S&P 500 components were up in two of the previous five trading days. After that, the S&P 500 rebounded about 13% before hitting a new low six months later, he said.The relief rally seen this week follows a difficult last month and quarter for the stock market, as the major indexes slid and jittery investors grew increasingly concerned about the Federal Reserve’s interest rate hikes and the probability of an incoming recession.Through the week’s first two trading sessions, the S&P 500 is up more than 5%. Tuesday was the best day for the S&P 500 since 2020.—CNBC’s Michael Bloom contributed to this report.