Re: Squeeeeeee!!!
I loved your Petunia almost immediately.
She was not, however, going to be ordered about by the old crackpot who had denied her the simple courtesy of being allowed to attend school with Lily, and the flourishing signature Professor Albus Dumbledore had her teeth grinding.
This definitely endeared her to me, as did her answer to Dumbledore's letter.
This said, you do a fantastic job at showing a realistic Petunia, not whitewashed but full of shades of grey. The structure of your story works really well: Showing only this special day throughout the years allows the reader to follow closely Petunia's change as well as to see her in a special light outside her usual surroundings. I love how you not only show her differences to Lily but also their similarities, how horse-faced Petunia can be beautiful, too.
I love the symbolism of the wilted lily blooming again. Especially the initial scene, when Petunia returns to the grave crying to see Snape doing his magic. Their bonding over this makes so much sense!
I love the slow buildup of their relationship and the way Petunia slowly starts to tolerate magic. Wesley is a wonderful OC to show this development.
Great use of the playground. Love that last scene between them in its urgency and despair!
And your Daisy Dursley must be the most original OC I've ever seen.