Diana attended a Gala Charity concert in London in 1981 to aid the Royal Opera House Development. She wore a long black taffeta gown that was sleeveless. It was a bustier style gown which means that her décolletage and shoulders were exposed.
Royal traditions for formal evening wear will usually require the shoulders to be covered but Diana went against the grain and selected a gown that not only showcased her shoulders but a bit of cleavage as well.
It was said that he scolded her because he thought the dress was too revealing.
Prince Charles could’ve also been jealous of the attention Diana received in the dress as there were quite a bit of male onlookers standing by the entrance that night. He also thought it to be a dress of “mourning” because it was black. She more than likely wanted to match his outfit which was black as well.
The dress was daring for a royal but Diana managed to pull it off very nicely !
Update: Thanks to all of you for upvoting and commenting on this answer. I really enjoyed reading your comments about Diana’s “Take the Plunge Dress.” If you’d like to know what happened to this gown, please view this answer: