• President Donald Trump's 25% tariffs on steel and aluminum imports take effect.
• The latest consumer price index report is due out Wednesday morning.
• Airlines say travel demand is waning, at least in the short term.

Here are five key things investors need to know to start the trading day:

1. Back and forth

Stock futures were higher in premarket trading Wednesday morning, extending a volatile week on Wall Street. On Tuesday the major indexes closed lower amid a back-and-forth on Canadian tariffs. The Dow Jones Industrial Average lost more than 1% on the day, and the Nasdaq Composite fell roughly 0.2%. The S&P 500, at one point notching 10% below its closing high, ended the session down about 0.75%. Follow live market updates Wednesday.

2. Man of steel

President Donald Trump's 25% tariffs on steel and aluminum imports take effect Wednesday — and Europe was quick to respond. The European Union said it would impose tariffs on more than $28 billion worth of U.S. goods, effective in April, as a countermeasure to what the European Commission called "unjustified trade restrictions." On Tuesday, Trump briefly planned to impose tariffs as high as 50% on steel and aluminum imports from Canada in response to a plan out of Ontario to establish a 25% tax on electricity exports to the United States. Ontario Premier Doug Ford and Trump both walked back their threats after some hours.

3. CP👀

Eyes out for CPI. The latest consumer price index report was released Wednesday morning and showed an increase of 0.2% in February. That put the headline inflation reading at 2.8% on an annual basis, lower than expected. While that result signals a steady drawdown in inflation from multi-decade highs, it's still well above the Fed's target.

4. Ceasefire plan

Ukraine has agreed to a 30-day ceasefire, provided Russia signs on. The plan, brokered by the U.S. during seven hours of negotiations in Jedda, Saudi Arabia, includes the U.S. lifting its pause on intelligence sharing and security assistance to Ukraine. Russia's initial response appeared hesitant: "You are jumping the gun, we have contacts scheduled with the Americans and then we'll see," Kremlin Press Secretary Dmitry Peskov told reporters Wednesday.

5. Cloudy outlook

Airlines say travel demand is waning, at least in the short term. American Airlines and Delta Air Lines both lowered their first-quarter forecasts this week, citing a weaker economic environment. The carriers have seen notable declines in leisure travel as well as government travel. "I think people are cautious and they're pulling back a little bit on travel, not in an organized manner but just kind of waiting to see what's going to transpire," Delta CEO Ed Bastian said at a conference.

– CNBC's Brian Evans, Katrina Bishop, Amala Balakrishner, Kevin Breuninger, Jeff Cox, Dan Mangan, Holly Ellyatt and Leslie Josephs contributed to this report.