The 1-day period is 1 day before the flight’s departure. The Order uses a 1-day time frame instead of 24 hours to provide more flexibility to the air passenger and aircraft operator. By using a 1-day window, test acceptability does not depend on the time of the flight or the time of day that the test sample was taken.

For example, if your flight is at 1pm on a Friday, you could board with a negative test that was taken any time on the prior Thursday.