<p>This study investigates the calcination temperature effects on the structural, elastic, thermodynamic, and optical properties of Mg_0.4Cd_0.3Cu_0.3Fe₂O₄ spinel ferrites synthesized via the sol-gel route and calcined at 850 °C and 950 °C. XRD analysis confirms that increasing the calcination temperature leads to an expansion of the lattice parameter and a growth in crystallite size. A corresponding enhancement is observed in the elastic parameters, including force constants, acoustic velocities, elastic moduli, stiffness coefficients, and Debye temperature. The calculated stiffness constants satisfy the Born stability criteria, and Pugh’s ratio indicates brittle behavior, while Poisson’s ratio of 0.25 suggests elastic isotropy. UV-VIS-NIR spectroscopy reveals that the sample annealed at 950 °C exhibits a reduced optical bandgap (<i>E</i>_g) and Urbach energy (<i>E</i>_u). Additional optical analyses (refractive index, skin depth, extinction coefficient, dielectric response, etc.) are presented. The reduced bandgap energies compared to pristine MgFe₂O₄ suggests that Cd/Cu co-substitution enhances visible-light absorption. The calculated optical parameters highlight the potential of Mg_0.4Cd_0.3Cu_0.3Fe₂O₄ ferrites for optoelectronic applications. Overall, the combined effects of optimized calcination temperature and cation substitution enable tuning of the structural and optical properties of Mg-based spinel ferrites.</p>