Charles Peckham Day (born February 9, 1976) is an American actor, writer, and producer. He is best known for playing Charlie Kelly on the FX dark comedy It's Always Sunny in Philadelphia (2005–present), which he stars with Rob McElhenney, Kaitlin Olson, Glenn Howerton and Danny DeVito, and he is also a writer and an executive producer. In 2011, he was nominated for a Critics' Choice Television Award and a Satellite Award for the role. He subsequently co-created the Fox sitcom The Cool Kids (2018–2019) with Paul Fruchbom and the Apple TV+ comedy Mythic Quest (2020–present) with McElhenney and Megan Ganz. He continues to executive-produce the latter.

In film, Day is best known for his performances as biologist Dr Newton Geiszler in Guillermo del Toro's science-fiction monster movie Pacific Rim(2013) and its sequel Pacific Rim Uprising (2018), Dale Arbus in the comedy Horrible Bosses (2011) and sequel Horrible Bosses 2 (2014), and teacher Andy Campbell in the comedy Fist Fight(2017). He is also known for his voice roles in Monsters University (2013), The Lego Movie film franchise (2014–2019) and the Nintendo franchise character Luigi in The Super Mario Bros. Movie (2023). He made his directorial debut with Fool's Paradise in 2023.

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