At first it started out as some sort of odd joke from Ron shortly before Harry's seventeenth birthday. Use the time turner to go back and fix anything & everything that they could. But as Harry kept thinking on the idea more & more. The more it actually made sense to it. Not only would he be saving his own parents, but he would also be saving possibly hundreds of other people that had died from the hands of Lord Voldemort & his followers.
Once everybody was convinced and a plan was made, the trio went back to the night before Harry's parents had died. They sent owls to several members of the Order, informing them of the pending doom that soon happen to each of them. Standing from the sidelines, they watched as everything they once knew began to change for the better.
It is now the summer before Harry's, Hermione's and Ron's final year at Hogwarts. Everything is how it should be. No Dark Lord. His parents are alive, Sirius and Lupin have families, he lives in Godric's Hollow with his siblings, and he's going to his seventh year at Hogwarts in September.