During the JUICE Lunar-Earth Gravity Assist (LEGA) period in August 2024, the JUICE ultraviolet imaging spectrograph (JUICE-UVS) performed a series of observations of the Earth and Moon, detecting reflected sunlight at the Moon and emissions of atmospheric species including hydrogen, oxygen, and nitrogen at the Earth. These observations provided the first opportunity for in-flight calibration of the instrument response to extended planetary targets. They were used to refine the wavelength calibration across the full instrument bandpass, confirm accurate knowledge of the pointing of the UVS field-of-view relative to the spacecraft, and validate previous measurements of the UV effective area determined from observations of UV-bright stars. The observations performed also demonstrate the range of scientific analyses to be performed during the science phase of the mission and are useful for the development and testing of relevant mapping tools and procedures. The JUICE-UVS LEGA data confirm that the instrument is in good health and well suited to its goals of characterizing the surfaces and atmospheres of Jupiter’s icy moons, mapping and monitoring Jupiter’s aurora and upper atmosphere, and studying the Jupiter-Io connection.