Tonight, Donny Osmond is on the ET stage as we go exclusively behind the scenes with Marie and the rest of the Osmond clan for their final public performance all together -- with a rare look backstage at what goes on before the show!

ET was the only show to get unprecedented access backstage at the Mormon Conference Center in Salt Lake City for the Osmond's last concert as a family -- an end to an amazing legacy.

"How do you put into words 50 years of experience? You can't do it," says Donny.

After five decades of performances, the Osmonds decided that they would not perform again as a complete group. With tears flowing, they performed their final concert with the Mormon Tabernacle Choir -- the first time that the choir has allowed cameras backstage.

"If you look right behind me, you'll see eight of the greatest men that I know," said an emotional Marie. "The only other great one has gone on with my mother, and I'm so proud of all of them. I'm so honored to be their sister."

Watch ET for more with the Osmonds!