He's Our Heroine Halloween Special

Though there might be a Halloween episode in season two which makes this non-canon. ;)

(Fun fact: the first Buffy novel after an adaptation of the pilot was about Halloween (and was later contradicted by the show's first Halloween episode) as was the second issue of the comic (which came out in time for Halloween a year after that episode, and that run has its own timeline issues but never mind).)

...

He's Our Heroine Halloween Special: Come As You Aren't

Halloween at Crestwood University, and despite Andy's attempts to claim he has too many assignments Lisa and Joe are taking him to a party at Donovan House.

"And since you haven't decided on a costume I've done that for you too." Lisa smiled playfully.
"... Of course you have..." Andy sighed deeply. "How trashy is it?"
"Oh, way less trashy than mine." Lisa grinned.
"That isn't very reassuring."

But the trashiness of his costume isn't Andy's only problem tonight! An unwelcome guest at the party means that old Halloween superstitions are coming true. Can Andy, Lisa and Joe save everybody in the house before the clock strikes midnight?