The first octopus landed on the ice during the Red Wings' 1952 Stanley Cup run, courtesy of brothers Pete and Jerry Cusimano, who owned a fish market. If you know your cephalopods, you will know that an octopus has eight tentacles. In those days it took eight playoff wins to claim the Cup, hence the supposed symbolism of the gesture.
The Red Wings were perfect in the '52 playoffs, sweeping the semifinal and the final in straight games. The octopus has been a good luck charm ever since.
By 1995, the team had adopted the tradition by introducing a mascot, Al the Octopus. Al is raised to the rafters of Joe Louis Arena before every home playoff game, and used in team merchandising and promotion.