During the rendering of honors, what event cues the side boys to salute in unison?

The event that cues the side boys to salute in unison is the boatswain's mate beginning to pipe. This is a significant action that signifies the start of the ceremonial honors being rendered, and the sound of the pipe serves as an auditory cue for the personnel involved in the ceremony.

When the boatswain's mate pipes, it indicates that an important individual, typically an officer, is either arriving or departing, and it marks the moment when side boys should execute their salute as part of the honors rendered. This practice underscores the Navy’s traditions and customs related to respect and protocol during formal events.

While the other actions, such as the flag being raised or the command "Attention," are important in their own right, they do not specifically trigger the simultaneous salute from the side boys as the piping does. The arrival of the officer is also important, but it is the sound of the piping that marks the precise moment of action for the side boys.