When the ladies in couture and the men in tuxedoes gather Monday night at the Metropolitan Museum of Art’s Costume Institute gala, two people were expected to be notably absent: Mayor Bill de Blasio and his wife, Chirlane McCray.

Mr. de Blasio and Ms. McCray sent their regrets to the event, as they did last year. An aide said Ms. McCray couldn’t go because she was being honored the same night by the Child Mind Institute, a nonprofit organization.

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