Day was a pioneer of materials chemistry, seeking unusual physical properties in inorganic and metal–organic compounds and models to explain them. He played a major role in the development of mixed-valence chemistry, and has carried out important and elegant experimental and theoretical work on the spectra, magnetic properties and conductivity of solid, inorganic complexes.
As a young researcher, he gave the first theoretically consistent description of the visible–ultraviolet spectra of vitamin B12 and its derivatives. Later, he put the assignment of inorganic charge-transfer spectra on a more rigorous basis; he correlated structures and physical properties of metal chain compounds and identified the first optically transparent ferromagnetic compounds by combined optical and neutron scattering methods. He also measured and systematised the optical properties of metamagnets.
Peter Day's graduate work initiated the study of mixed-valence compounds and led to the Robin-Day Classification of such species.
In 2012, he published a memoir about his life in science.