Yellen is poised to deliver a speech at 1.30 p.m ET regarding the state of the jobs market in the U.S. Earlier this month, the central bank raised interest rates and is expected to do so again next year.
The yield on the benchmark 10-year Treasury notes, which moves inversely to price, was lower at around 2.551 percent, while the yield on the 30-year Treasury bond was also lower at 3.136 percent.
In oil markets, Brent crude traded at around $55.17 a barrel midmorning Monday, down 0.04 percent, while U.S. crude was around $52.02 a barrel, up 0.12 percent.
Investor optimism appeared to be increasing that OPEC and other oil producers would be able to tighten crude supply and prop up prices in 2017.