via Ryan Rishaug tweets,

NHL players need 3 negative tests spaced 48 hours apart in the 7 days prior to travel to Canada. Any positive tests after Wed or Thurs this week and they won’t hit that mark. We could be talking about players left behind until cleared. Crucial week for the leagues testing.

Any positive tests, teams would likely re-test immediately to rule out false negatives, but each test has turn around time, and there is little margin for error. Have heard stories of teams testing twice the required amount the last 10 days or so to make sure they’re good.