This Saturday, 23rd February, research rewards will grant the long-awaited Clamperl for three hours only (10am to 1pm here in the UK). The clam-like Pokémon is one of the last left unreleased from Gen 3, skipped at the time because of its tricky evolution mechanic.
As with Feebas Day, you'll be catching Clamperl by completing research tasks, rather than by finding it in the wild.
Unlike with Feebas Day, themed spawns will provide increased chances of catching water-type Pokémon like Krabby and Wailmer (both Shiny-possible) as well. Double Stardust for water-type catches will also be in effect. And by hunting down a new Pokémon to gain candy for its two evolutions, there will be less of the disappointment seen on Feebas Day when finding yet another non-shiny.
There's no word on whether Clamperl itself will be Shiny, although Niantic has since clarified how it is handling Clamperl's evolution mechanic - and that the Pokémon will be available in the wild afterwards if you can't make the event.
In the regular Pokémon games, Clamperl will evolve into either Huntail or Gorebyss depending on which one of two evolution items it is holding. In Pokémon Go, its evolution will simply be random (which saves on bag space for unwanted evolution items, but may require you trade if you get too many duplicates).
Key to this event being a success, in my mind, is how straightforward the research tasks will be - and that's something we won't find out until the day. But with Clamperl needing plenty of candy for evolutions and Niantic openly saying it has listened to feedback from Feebas Day to revamp the concept, it seems likely things will be a lot more manageable this time around. Here's Pokémon Go community manager Liz George, responding to a fan on Twitter last night: