Connick, Jr. will appear as a guest mentor and help the finalists — Crystal Bowersox, Lee DeWyze, Casey James, Aaron Kelly and Michael Lynche — prepare to sing the music of Sinatra (whose music is currently featured on Broadway in Twyla Tharp's Come Fly Away).

The singer-songwriter will also mark the first time an "American Idol" mentor will arrange the songs, write the orchestrations for the contestants and perform, according to FOX. Connick, Jr. will sing on the May 5 results show. Pop songstress Lady Gaga will also perform.

The hour-long Sinatra-themed "American Idol" will air May 4 at 8 PM and May 5 at 9 PM. (All times ET live/PT tape-delayed.)

Crooner Connick Jr. will return to Broadway this summer (July 15-26) for an 11-performance concert engagement at the Neil Simon Theatre. The Tony Award nominee for his turn in the Broadway revival of The Pajama Game also earned a Tony nom for Best Score to his musical Thou Shalt Not. The three-time Grammy Award winner has also appeared in the films "Hope Floats," "Little Man Tate," "Copycat," "Independence Day" and "P.S., I Love You" as well as on television's "Will & Grace."

For more information about "American Idol", visit americanidol.com.