WASHINGTON—U.S. President Joe Biden set out to turn the tables on the COVID-19 pandemic Thursday with a 10-point winter offensive against the Omicron variant that further complicates flying from Canada to the United States on the eve of the holiday travel season. As early as Monday, Canadians and all other foreign visitors who travel to the U.S. by air will need to get a COVID-19 test no later than one day before their departure. Biden is slashing the testing window—currently three days for fully vaccinated travellers—as part of a suite of public health measures aimed at slowing and limiting the spread of a highly mutated variant about which there are more questions than answers. “All inbound international travellers must test within one day of departure, regardless of their vaccination status or nationality,” Biden said as he outlined the plan at the National Institutes of Health headquarters in Bethesda, just north …