U.S. stocks advanced in the lightest day of trading so far this year.

Monday’s broad gains wiped out Friday’s losses, which came after Federal Reserve Chairwoman Janet Yellen said that the case for an interest-rate increase had improved.

But some investors are skeptical that an increase will come soon, or that it would be enough to shake the long-running rally.

“The bull market ends on a recession, it doesn’t end on a rate hike,” said Michael Antonelli, equity sales trader at Robert W. Baird.

Trading activity died down after Friday’s intense focus on Ms. Yellen’s comments. Roughly 4.9 billion U.S. shares changed hands Monday, the lowest volume since Dec. 30. The daily average for 2016 is around 7.4 billion.