OK, technically the goal was scored very early in the morning on May 5, 2000 -- but we'll let that slide since the game actually started on May 4.
Game 3 between the Flyers and Penguins back in the 2000 Eastern Conference Semifinals was a key one. Philadelphia was the No. 1 seed in the East, but had lost Games 1 and 2 to the Penguins at First Union Center.
They won Game 3 in one single overtime, but needed another victory in Game 4 to realistically keep the series in hand.
Seven-and-a-half periods later, enter Keith Primeau, who saved things with the huge fifth overtime tally. It was the third-longest game in NHL history, and it remains the longest in modern NHL history. The Flyers went on to roll Pittsburgh 6-3 back in Philly in Game 5, and then they wrapped the series in Game 6 with a 2-1 win.
They later fell to the New Jersey Devils in a seven-game Eastern Conference Finals series that we'll never speak of again.