"My Bloody Valentine," "Supernatural"
It's seems only natural to talk about "Supernatural" first, and this Season 5 episode is a doozie. With the help of Castiel, Sam and Dean investigate a string of deaths -- all of which involve couples who eat each other to death. Things escalate to affect even those who are single, as more and more people start binge eating or drinking themselves to death. The boys theorize Famine, one of the Four Horsemen of the Apocalypse, is behind all these strange occurrences. All the while, poor Sam is trying to beat his addiction to demon blood, which makes this episode bloody good fun.
"Bewitched, Bothered, and Bewildered," "Buffy the Vampire Slayer"
In the Season 2 episode, Cordelia dumps Xander on Valentine's Day. Deeply hurt by her betrayal, Xander teams up with teen witch Amy to cast a love spell on Cordelia. But the spell backfires, and Cordelia isn't affected -- but every other girl and woman in Sunnydale is. Meanwhile, Buffy still tries to get over Angel's recent transformation, and Giles warns her that Angelus can be especially vicious around Valentine's Day. As for Angel, he's busy trying to steal Dru away from Spike, but even Dru can't resist Xander's love charm. Even Buffy's mom finds herself drawn to Xander, a move that totally freaks both him and Cordelia out.
"Animal Pragmatism," "Charmed"
When three of Phoebe's classmates transform a snake, a pig, and a rabbit into hunky men (thanks to a book of love spells and a tape recording of Phoebe's voice), things quickly turn violent. Two of the animals-turned-men decide they don't want to go back, and so they kidnap two of the girls and go on the hunt for Phoebe. Meanwhile, Piper has two Valentine's Day suitors, but Prue is alone (and to add insult to injury, she's also freshly unemployed). The ending has a lovely sense of poetic justice.
"The Rain King," "The X-Files"
The story begins on Valentine's Day, when Sheila Fontaine and Daryl Mootz get into an argument because she had the newspaper publish their engagement announcement. He drives off in a rage, and then heart-shaped hailstones total his car. While Mootz is thought to be the rainmaker, the revelation of who is behind the rain may surprise you. Hint: It's got a lot to do with unrequited love. But thankfully, it's got a happy (if somewhat sappy) ending.