On the afternoon of that blustery Valentine's Day, tiny red boxes with tiny silver bows appeared before every female witch on the British Isles. Witches the island over were compelled to open the adorable boxes; The twins would forever denied any sort of actual compulsion spell on the boxes, but not one was left unopened. When the boxes were opened, a ball of intense pink magic flew out and circled the witch in question. After one lap around said witch's head, the spell latched onto the witch in some way and quickly faded from sight. Almost as one, the entire magical female population of Brittain developed a brief smitten look, which lasted from seconds to in one unique case three weeks.
There were a few exceptions, naturally.
Dolores Umbridge's ball of energy turned a dingy grey for a brief moment before flying away from her in revulsion. It instead attached itself to a nearby dementor. The creature withered away, twitching, and the esteemed Ms. Umbridge was forever sure there after that the box in question was some sort of assasination attempt gone luckily astray.
One Bellatrix Lestrange opened her box, only to be nearly choked by the tiny ball of magic as it dived down her throat. As the magic quickly did its will, her whole body jerked several times, and she began doing something she'd never quite managed before; She smiled a sunny smile and hummed a happy tune. Despite earning herself several crucios, and quite a few other dark curses, the smile didn't leave her face for weeks.
Granted those were unique cases. By and large the magic took on four basic 'forms' based on the recipient. Almost all young witches got the same reaction from the magic. It planted itself right on their forehead, giving the sensation of being given a great big hug and a kiss on the forehead, and leaving behind a very cool pair of color shifting lip prints that lasted until one washed their face.
Another large section of the female population, including most married or widowed witches, had the magic touch their right hand. These ladies, including Madam Augusta Longbottom, all later claimed to have had their hands taken and kissed by a ghostlike bowing figure, which thereafter disappated. They were left holding a single white rose.
Another, smaller group of witches; including Molly Weasley, Minerva McGonagall , Hermione Granger, Fleur Delacour, Ginny Weasley, and several other women known to be near and dear to Weasley hearts were enveloped in a tight hug and recieved a kiss on the cheek. They were left with a small bouquet of white roses.
The final two witches to recieve their Valentine's were Katie Bell and Angelina Johnson. Both girls were touched by the magic, and left with flushed cheeks and a very satisfied smile. They also found themselves each holding a dozen red roses and an invitation, which including a portkey to the twin's personal quarters above their Diagon Alley store.
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Disclaimer: I do not own Harry Potter, nor either of the Weasley twins, unfortunately.