It happened on May 17, in NHL Stanley Cup Playoff History

May 17, 1981
In Game Three of the 1981 Final, Minnesota North Stars right winger Dino Ciccarelli broke Don Maloney’s one-year rookie playoff scoring record (20 points in 1979) with his 21st point, a goal against the New York Islanders. The Islanders won the game 7-5 and later took the series 4-1.

May 17, 1983
The New York Islanders beat the Edmonton Oilers 4-2 to complete a four-game sweep of the 1983 Final. It was the Islanders’ fourth straight Stanley Cup, one short of the NHL record for consecutive championships set by the Montreal Canadiens from 1956 to 1960.

Source: NHL Media Release