What do you call this event in the picture?

A Masquerade Ball is an event in which many participants attend in costume wearing a mask. Less formal "costume parties" may be a descendant of this tradition. A Masquerade Ball usually encompasses music and dancing. These nighttime events are used for entertainment and celebration.

Masquerade Balls were a feature of the Carnival season in the 15th century, and involved increasingly elaborate allegorical Royal Entries, pageants, and triumphal processions celebrating marriages and other dynastic events of late medieval court life.

These events were extended into costumed public festivities in Italy during the 16th century Renaissance. They were generally elaborate dances held for members of the upper classes. and were particularly popular in Venice.

One of the most noted masquerade balls of the 20th century was held at Palazzo Labia in Venice on September 3, 1951, hosted by Carlos de Beistegui. It was dubbed "the party of the century".

Another famous ball was the 'Black and White ball'. It was held on November 28, 1996 at the Plaza Hotel in New York City. Hosted by author Truman Capote, the ball was in honor of the Washington Post publisher, Katharine Graham.