Jade Williams (born May 21, 1988 in Sidcup, England, calls Broxbourne home) is a British solo pop singer, who has performed under the stage names Sunday Girl and Whinnie Williams. After being asked (by Claire Pitcher) about the name "Sunday Girl", Jade was quoted as saying, "In Broxbourne theres this little old fashioned pet shop called Paws and Claws and I worked there at the weekends. The staff there always used to refer to me as the Sunday Girl; it became my nickname. I liked how it was quite anonymous, so it just stuck as a stage name."

Sunday Girl's debut single Four Floors was released through Polydor Records on 28 March 2010. The song was co-written and produced by Diplo. A music video directed by Elisha Smith-Leverock was also released to accompany the release of the single. On 15 July 2010, she released her second single (and video) Self Control originally performed by Laura Branigan.

Jade supported Ellie Goulding on tour during November 2010, and played her first headlining show at London's XOYO on 15 December 2010. She (with Jim Eliot of Kish Mauve) wrote and produced her third single Stop Hey, released in February 2011. She co-wrote and performed Let's Not Play Games for Martin Solveig's 2011 album Smash.

Her cover of Love U More (originally by Sunscreem) was promotionally released along with its accompanying video in October 2011, but its commercial release was delayed indefinitely. That November, she also went on tour to support LMFAO on a few of their UK dates. Williams then recorded a cover of Pixies' Where Is My Mind? for Thomson's 2011 holiday TV campaign, and the single's success helped her to be signed by Universal's Island Records in early 2012. High & Low was released on the official motion picture soundtrack of StreetDance 2.

In February 2013, Williams announced the end of the Sunday Girl project. She released her first Whinnie Williams single and music video You Don't Love Me in early 2013.