DISCLAIMER: Harry Potter is not mine.
"You didn't just lose a mother that night in Godric's Hollow, you know. I lost a sister." -Harry Potter and the Deathly Hallows: Part I Deleted Scene
Petunia and Lily, growing up, had loved Halloween. They had always helped each other make costumes, though that had stopped when Lily got her Hogwarts letter.
Their parents had, in an attempt to get Petunia excited about her sister's time at Hogwarts, had read aloud the letter Lily had sent- by an owl, of all things- describing the Halloween Feast at Hogwarts, though Petunia had simply made a biting retort about that freak school and moved on with her life, dressing up as a 1920s flapper girl and going out with her friends.
The next year, Petunia isn't up to celebrating Halloween. Lily's letter is almost five pages long, as always, but, according to their father, she's just rambling the entire time about Severus and some Potter bloke that she doesn't give the first name of. Their entire family is on edge.
Their mum dies on 1 November of that year.
For the next two years, Petunia celebrates Halloween the way she always had: she dresses up and goes out with her friends. Once she meets Vernon, her enthusiasm for the Halloween starts to wane as she realizes that she's an adult now and doesn't have time for such nonsense.
She tells Vernon that her sister is a witch on Halloween. It's only fitting, really.
When she opens the door that fateful 1 November of 1980, she shrieks. But every Halloween from then on is changed.
One year, she picks Harry and Dudley up from school on a Halloween dress up day and sees a little girl with red hair dressed as a witch.
She lets Harry and Dudley play for about an hour more so she can cry without Vernon noticing, because she may have been a freak, but Lily Potter was still her sister, and she will always mourn for her sister.