Pokémon GO blog had to say about Marill Limited Research Day:
As Fairy-type Pokémon continue to appear more often in the wild during the Luminous Legends X event, you can look forward to a Limited Research event featuring the Water- and Fairy-type Pokémon Marill! On Sunday, May 9, 2021, from 8:00 a.m. to 10:00 p.m. local time, you can complete event-exclusive Timed Research tasks that lead to encounters with Marill. If you re lucky, you might even encounter a Shiny Marill! You ll also be able to complete event-exclusive Field Research tasks that lead to encounters with Marill.
For those who haven t played Limited Research Days before, they generally offer extremely high Shiny odds. With the exception of Community Days and Raid Days, these have historically seen the biggest boost to Shiny rates in the game. This is temporary of course, so the special green variant of Marill will only be easier to get during these hours.