Summary: Over the years, Harry had come to realize that the best time to get answers out of Aunt Petunia was after a good book club meeting when she was full of Chardonnay, good food and in a contemplative mood. He'd have to time it right, though, as Uncle Vernon and Dudley couldn't be home. This singular mood is how he found out his parents names. How he found out his birthday, and other such important information as when next Aunt Marge would be visiting so he could be scarce.
Once he started to go to Hogwarts, these info sessions got much more interesting. Here is how he found out that his father was a bully, that he was JUST like his mother in his actions and that his aunt couldn't stand him because of it. It was also in one of these sessions that he happened to mention Professor Snape. What he found out changed his life and his perceptions of the world as he knew it dramatically.
CHALLENGE: A story in which Harry finds out Snape's background from Petunia during one of these 'info sessions'. He found out that Snape grew up and was friends with his mother, his father was an alcoholic muggle that abused him, Harry's father bullied him at school, and his mother was a witch (along with any other perceptions Aunt Petunia chooses to share). This can happen in the year of your choosing.
How does this change Harry and Snape's relationship? Does Harry confront Snape? Maybe yell back at him that he's just like his father after one too many times of being compared to his own?
Bonus points for a really good characterization of a Chardonnay soused and chatty Petunia.