Rihanna's Radio City Rehearsal

Rihanna arrives for a full run-through rehearsal at Radio City Music Hall in New York City on Thursday morning.

Tomorrow, the 20-year-old Barbadian beauty will join Carrie Underwood, Ashanti, Ciara, Fergie, Keyshia Cole, Leona Lewis, Mariah Carey, Mary J. Blige, Miley Cyrus, Natasha Bedingfield, and Sheryl Crow in a live performance of “Just Stand Up,” the unprecedented charity single in support of Stand Up To Cancer (SU2C).

The Stand Up To Cancer TV special will be airing simultaneously on ABC, CBS, and NBC on Friday, September 5th. Rihanna will also perform her hit singles “Take A Bow” and “If I Never See Your Face Again” on stage at Fashion Rocks, which airs the following Tuesday night, September 9 in a two-hour special @ 9PM ET/PT on CBS.