Here we report a theoretical analysis of the luminescence properties of Sr₂Si₅N₈ host lattices codoped with Ca²⁺ and Eu²⁺. These systems have been first synthesized by Li et al. [J. Solid State Chem., 2008, 181, 515], who have found that Ca²⁺ doping provokes a red-shifting of the emission peak of Eu²⁺, from 620 nm to 643 nm. However, the mechanism that drives this shift is still unclear from experimental data. Based on density functional theory and ligand field analysis, we study the structure, stability, and emission properties of Eu²⁺ embedded in the (Sr_1−xCa_x)₂Si₅N₈ host lattice. Our results provide a full explanation of the experimental data and the methodology could constitute a valuable tool for the design of phosphors with tunable emission spectra.