Be on the lookout for more Clefairy, Jigglypuff, Slowpoke, Chansey, Porygon, Flaaffy, Hoppip, Snubbull, Miltank, Whismur, and Skitty than usual. If your Pokédex has been sorely lacking when it comes to the pink Pokémon, this event is the best time to fix that. You might also see a Shiny Chansey in the wild or hatch a Shiny Happiny.
Source : https://www.distractify.com/p/valentines-day-pokemon-go