Attack at the Great Lake

The Attack of the Hogwarts Lake took place when over a hundred dementors attacked Harry Potter, Hermione Granger, and Sirius Black at the Hogwarts Lake in 1994. Harry Potter attempted a Patronus Charm, but failed and was saved by his future self through a Time Turner. He woke up in the hospital wing, and with Hermione he tried to convince Cornelius Fudge, Severus Snape, Poppy Pomfrey, and Albus Dumbledore that Sirius was innocent. Only Dumbledore believed them. The Headmaster, then, discretly toldthem to go back in time. They oblidged. Once they rescued Buckbeak the Hippogriff, they went down to Rubeus Hagrid's hut to hide from Remus Lupin the Werewolf. Harry, believing his late father had conjured the Patronus, headed down to the lake to watch. Realizing that he had created the Patronus, He screamed the incantation Expecto Patronum and warded off the dementors and flew down to rescue Sirius.

Later, the next year, the future members of Dumbledore's Army used this to get Harry to teach them Defence against the Death Eaters, during the Second Wizarding War.