I sing backups for an Elvis impersonator (and the Garth Brooks, Cher and Patsy impersonators who travel with us). It's like a circus. Also I sang lots of soul/R&B nonsense with many folks, and I sang with the Reactions (who turned into the Immaculate Machine). I wish I was Otis Redding (although not dead).
For physical events that happen at a specific time. For example a concert, or dance performance. If there are multiple shows, you can still duplicate your event to cover them all.