Niantic has announced that from Wednesday, April 24th, 2019 at 1:00 p.m. PDT until Sunday, May 5th, 2019 at 1:00 p.m., players have a chance to encounter Shiny Meltan.

Shiny Meltan returns for a limited time!

Trainers, We are excited to announce that Shiny Meltan is back for a limited time! Meltan, a mysterious and rare Pokémon, only appears when the Mystery Box is opened in Pokémon GO. If you’re lucky, you may encounter Shiny Meltan when you open the Mystery Box from Wednesday, April 24, at 1:00 p.m. PDT until Sunday, May 5, at 1:00 p.m. PDT!

The key to catching Meltan is the Mystery Box, which can be obtained by sending a Pokémon from Pokémon GO to the Nintendo Switch titles Pokémon: Let’s Go, Pikachu! or Pokémon: Let’s Go, Eevee!

Until now, Trainers had to wait seven days after opening the Mystery Box before opening it again. Starting Wednesday, April 24, at 1:00 p.m. PDT, you’ll be able to open the Mystery Box once every three days, making it easier to get Meltan! Don’t miss your chance to evolve Shiny Meltan into Shiny Melmetal! Start collecting those Meltan Candies, Trainer!

Date + Time

• Wednesday, Apr. 24, at 1:00 p.m. PDT (GMT −7) until Sunday, May 5, at 1:00 p.m. PDT (GMT −7)

Features

• When you open the Mystery Box, you may encounter the Shiny version of the Mythical Pokémon Meltan!
• Trainers will now be able to open the Mystery Box every three days, even after the event ends.

Source: https://pokemongolive.com/en/post/shinymeltanreturns/