When will a commercial flight of a Boeing 737 MAX aircraft next take off in the United States?
After two fatal crashes in five months, regulators around the world grounded Boeing's 737 MAX aircraft in March 2019 pending further investigations (NPR). While U.S., EU, and Canadian regulators have completed test flights key for recertification, it is not known when commercial flights will resume (ABC News, CNN, FAA).
NOTE 12 October 2020: For the purposes of this question, "commercial flights" are flights with ticketed passengers. Cargo only flights would not count.