Buffy the Vampire Slayer
A full list of special holiday episodes

Follow Buffy Summers as she balances being a teenage girl and the chosen one who must fight supernatural forces to save the world.
The show aired 1 Christmas episode, 1 Thanksgiving episode, 2 Halloween episodes, and 1 Valentine's Day episode.
Christmas episodes:

Season 3, Episode 10
Amends
As Christmas approaches, Angel is plagued by malevolent visions that urge him to eliminate Buffy.
Thanksgiving episodes:

Season 4, Episode 8
Pangs
On Thanksgiving, Buffy comes across the spirit of a Native American warrior whose people were annihilated by settlers.
Halloween episodes:

Season 2, Episode 6
Halloween
In this Halloween-themed episode, Buffy and her friends visit a costume shop where the costumes seem to have a life of their own, turning customers into the characters they're dressed as.

Season 4, Episode 4
Fear Itself
Buffy and her friends become trapped inside a haunted fraternity house on Halloween, where they encounter real bats, zombies, and a terrifying demon.
St. Valentine's Day episodes:

Season 2, Episode 16
Bewitched, Bothered and Bewildered
Xander tries to improve his love life through witchcraft, but his spell goes wrong and he finds himself with multiple girls competing for his affection.