On Saturday, November 28, from 2:00 p.m. to 5:00 p.m. local time, Mega Clefable makes its Pokémon GO debut during Clefable Super Mega Raid Day.
You will meet it in Super Mega Raids, which take a group and only appear at certain Gyms. Clefable caught there come with Mega Level 1 already unlocked, between 1,322 and 1,392 CP (1,653 to 1,741 in cloudy weather), and your chance at a shiny Clefable goes up during the event. Clefable is a Fairy-type: Poison and Steel attacks hit it harder.
The whole event is in our article and in the official announcement.