The European Space Agency’s JUpiter ICy moons Explorer (JUICE), launched in April 2023, aims to assess the potential habitability of Jupiter’s icy moons–Ganymede, Europa, and Callisto–while investigating the Jovian system as a whole. During its cruise phase, JUICE performed in August 2024 the first-ever combined Moon–Earth gravity assist (LEGA), a critical milestone for optimizing its trajectory toward Jupiter. Beyond its navigation objectives, this unique event provided an exceptional opportunity for in-flight calibration and validation of the scientific payload under known Earth environment. Despite significant operational constraints, all instruments were operated. The Moon flyby enabled precise calibration of remote sensing instruments, radar testing and in situ measurements of the Moon’s environment while within the Earth’s magnetotail, as well as the detection of a spacecraft outgassing event observed by multiple instruments and sensors. The subsequent Earth flyby complemented these efforts through atmospheric, magnetospheric, and radiation belt observations. These measurements confirmed the excellent performances of the JUICE instruments and demonstrated the mission’s capability to deliver high-quality scientific data already during cruise. This campaign represents a key step in preparing for nominal operations at Jupiter while providing valuable scientific results.