Near 9:43 a.m. ET, the Dow was up 24 points (+0.14%), the S&P 500 was up 4 points (+0.2%) and the Nasdaq was up 10 points (+0.21%).
The long Memorial Day weekend is just ahead, and volumes are typically lower on a day like this as some on Wall Street get a head start.
But it's been a strong week that pushed markets closer to all-time highs. The S&P 500 is headed for a five-day advance of at least 2%.
Earlier, the second estimate of Q1 GDP showed that the economy grew 0.8%, revised up from the 0.5% pace previously reported. The improvement was mostly due to a reduced drag from inventories, although with final sales unchanged, stockpiles are still elevated according to Deutsche Bank's Joe LaVorgna.
The University of Michigan's consumer sentiment for May was 94.7, revised down from earlier in the month, but up from 89 in April.
At 1 p.m., Baker Hughes will publish its weekly count of oil and gas rigs.
Then at 1:15 p.m. ET, Yellen is expected to begin a conversation with Harvard professor Greg Mankiw. As usual, markets will be looking for comment on what she's thinking about monetary policy, and this time, on the odds of a rate hike in June.
After cracking $50 per barrel on Thursday, West Texas Intermediate crude retreated and was down 1.5% to $48.73.