Losses for the US dollar against a basket of major currencies came as data showed US economic growth was a bit more sluggish than initially thought in the second quarter.
Investors were wary of Yellen hinting at a near-term interest rate hike when she speaks, which could divert some of the liquidity that has underpinned riskier assets worldwide, though others predicted she would strike a more equivocal note.
Bank of Canada Governor Stephen Poloz is attending the annual economic policy symposium in Jackson Hole at which Yellen is speaking, but will not have a speaking role, a spokesperson for the central bank said earlier this week.
US crude prices were unchanged at $47.33 a barrel. Oil fell earlier in the day after the Saudi energy minister watered down expectations that the world's largest producers might agree next month to limit their output.
At 9:23 a.m. EDT (1323 GMT), the Canadian dollar was trading at C$1.2881 to the greenback, or 77.63 US cents, stronger than Thursday's close of C$1.2926, or 77.36 US
The currency's weakest level of the session was C$1.2922, while it touched its strongest since Tuesday at C$1.2862.
Canada will likely maintain its 2 percent inflation target and bypass alternative policy goals when the central bank renews its inflation-control agreement this year, strategists say, but the main measure of core inflation may change.
Canadian government bond prices were slightly higher across the maturity curve, with the two-year bond up 1.5 Canadian cents to yield 0.584 percent and the benchmark 10-year rising 9 Canadian cents to yield 1.056 percent.