https://thehockeywriters.com/hockey-history-april-1-bossy-gretzky-howe-hall-hossa-stevens/
On April 1, 1919, the sixth and final game of the Stanley Cup Final
was canceled because of an influenza epidemic. The series was a rematch
of the 1917 Stanley Cup Final between the NHL champion Montreal
Canadiens and the Seattle Metropolitans of the Pacific Coast Hockey
League.
The Metropolitans won Games 1 and 3 before Game 4 was
considered a tie after no goals were scored through two overtime
periods. The Canadiens won Game 5 in overtime to even up the series at
2-2-1. Game 6 was canceled less than six hours before it was scheduled
to begin as multiple players on both teams became ill. Canadiens player
Joe Hall died of pneumonia brought on by the flu, four days later.
Originally the Canadiens wanted to forfeit the Cup to Seattle because
they couldn’t dress a full lineup, but the manager-coach of the
Metropolitans, Pete Muldoon, refused to accept it because the illness
caused the shortage of players. No winner was ever declared, marking
this as the only time the Stanley Cup was not awarded after the playoffs
had begun.