President Trump signed the US-Canada-Mexico Agreement – USMCA – into law on January 29, 2020. While the legislation has been passed, the U.S. must still write the implementing regulations. Mexico has already ratified the agreement, while Canada has not yet done so.
Once all three parties ratify the agreement and implement uniform rules, a formal effective date could then be announced.
April 2020 update:
The U.S., Canada and Mexico have all ratified the new US-Canada-Mexico Free Trade Agreement (USMCA).
The earliest the new measures could take effect is July 1.
For more information, see https://ustr.gov/usmca