It happened on May 19, in Stanley Cup Playoff History

May 19, 1974
The Philadelphia Flyers, who had entered the NHL in 1967-68, became the first expansion team to win the Stanley Cup, downing the Boston Bruins 1-0 in Game Six of the 1974 Final. Left winger Rick MacLeish scored the game’s only goal, while goaltender Bernie Parent, who won the Conn Smythe Trophy as playoff MVP, recorded the shutout.

May 19, 1984
The Edmonton Oilers, one of four former WHA teams which joined the League in 1979-80, won their first Stanley Cup title. Oilers center Mark Messier, who registered an 8-18-26 scoring mark in 19 playoff games, won the Conn Smythe Trophy.

May 19, 2007
Captain Daniel Alfredsson scored at 9:32 of overtime to give the Ottawa Senators a 3-2 victory over the Buffalo Sabres and a 4-1 series victory in the Eastern Conference Final. The win put the Senators in the Stanley Cup Final for the first time since joining the NHL in 1992-93 and gave the city of Ottawa’s its first Cup Final since 1927.

Source: NHL Media Release