"We know how prominent the 2,800 level has been, and it goes way beyond the last few days," Frank Cappelleri, executive director at Instinet, wrote in a note. "In fact, this now is the 11th visit to the 2,800 zone since the start of 2018."
"As we saw last summer, once 2,800 was officially overtaken, it became support. And that support helped propel the market to new highs," he said.
Friday's moves come a day after the major indexes posted solid monthly gains in February, pushing the S&P 500 to its best start to a year since 1991. The S&P 500 is up more than 11 percent for the year, along with the Dow. The Nasdaq, meanwhile, is up more than 14 percent.
This could mean further gains for investors as the year goes on. Data compiled by LPL Financial show that, in 25 of 27 occasions since 1950, the S&P 500 has posted gains in the final 10 months after rising in January and February.
"We think equities have some room to go higher," said Jeroen Blokland, a portfolio manager at Robeco. But "I don't think we get another rally like that. It's going to be a little bit more difficult. It could happen, but a lot of things would have to go right."