David Letterman will be leaving his late-night talk-show chair for good on May 20. And as of right now, there are only 28 shows left for him after 33 years as host of an after-hours TV program, first with Late Night With David Letterman and then Late Show With David Letterman for the past 22 years.

He's starting to round up his final guests for those last shows, and some familiar faces will join Letterman before he says goodbye. Both Tom Waits and Elvis Costello have been tapped as artists who will be among his final musical guests next month.

According to BillboardMumford & SonsDave Matthews Band, Norah Jones and the Avett Brothers will perform sometime during the last couple dozen shows too. George Clooney, Robert Downey Jr., Tina Fey and Will Ferrell will also appear.

More special guests are promised to be announced later.

Waits has appeared on Letterman's shows many times over the years, always making for a hilarious and chatty guest. And Costello memorably filled in as a guest host for Letterman back in 2003, when the late-night host was recovering from an eye infection.

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