An American actress, writer, and web series creator. She is the creator, star, writer, and producer of the original web series The Guild (2007–2013), a show loosely based on her life as a gamer. She also wrote and starred in the Dragon Age web series Dragon Age: Redemption (2011). Day was a member of the board of directors of the International Academy of Web Television beginning December 2009 until the end of July 2012.
On television, Day has played Vi in the series Buffy the Vampire Slayer (2003) and Dr. Holly Marten in Eureka (2011), and had a recurring role as Charlie Bradbury on Supernatural (2012–2015, 2018–2020). She has also acted in movies such as Bring It On Again (2004), as well as the Internet musical Dr. Horrible's Sing-Along Blog (2008). In April 2017, she began appearing as Kinga Forrester in Mystery Science Theater 3000.