Peter Day was born in 1938 in Kent and educated at the local village primary school and nearby grammar school at Maidstone. He was a Scholar, and subsequently a graduate student at Wadham College, Oxford, of which he is now an Honorary Fellow. His doctoral research, carried out in Oxford and Geneva, initiated the modern day study of inorganic mixed valency compounds. From 1965 to 1988 he was successively Departmental Demonstrator, University Lecturer and Ad Hominem Professor of Solid State Chemistry at Oxford, and a Fellow and Tutor of St John's College, to which he was elected an Honorary Fellow in 1996. Elected Fellow of the Royal Society in 1986; in 1988 he became Assistant Director and in 1989 Director of the Institut Laue-Langevin, the European high flux neutron scattering centre in Grenoble. In 1991, he was appointed Director of The Royal Institution and its Davy Faraday Research Laboratory, where subsequently he became Fullerian Professor of Chemistry. In 1994, he received the honorary degree of Doctor of Science from the University of Newcastle upon Tyne, in 1999 from the University of Kent at Canterbury and in 2002 he became an Honorary Fellow of University College London. He has held visiting appointments at Universities in Australia, Denmark, France and Spain, and at the corporate research laboratories of Bell, IBM, Xerox and American Cyanamid.

Peter Day has served on many Royal Society and UK Research Council committees, the British Council Science Advisory Committee and the Engineering and Physical Science Core Group of the European Science Foundation. International honours include membership of the Academia Europaea, of which he is the Treasurer and a Trustee, and the Indian Academy of Sciences. In Britain he gave the 1999 Bakerian Lecture of the Royal Society, the Society's premier award lecture in the physical sciences, as well as the Society's Blackett and Humphry Davy lectures. While from the Royal Society of Chemistry he received the Corday-Morgan Medal and the Award for Solid State Chemistry. He has advised the European Commission, the French Ministry of Education, the Portuguese and Swiss Ministries of Research and the Swiss National Science Foundation. He has strong contacts with the academic and business communities in Japan, having acted as external Counsellor to the Institute for Molecular Sciences in Okazaki, and External Assessor of the School of Materials Science at the Japan Advanced Institute of Science and Technology, Kanazawa. In 1998 was awarded the Daiwa-Adrian Prize, together with colleagues in Oxford and Yokohama for collaborative research on organic magnets.

Over nearly 40 years Peter Day has led the worldwide search for new patterns of physical behaviour in inorganic and metal-organic compounds, and the chemical models to understand them.

He was one of the first to recognise the potential of molecular crystals as conductors or magnets, and his definitive experiments in the 1960's on photoconducting phthalocyanines led to a family of gas sensors. Shortly after, his ground breaking work at Oxford and Bell Laboratories revealed mixed valency compounds as a distinct class of inorganic substance with their own characteristic properties, which he successfully rationalised in a scheme still widely cited 35 years on, extensively quoted in undergraduate textbooks. In parallel, he was also the first to show how definitive information on the ordering of molecular orbital energy levels in metal complexes, particularly of tetrahedral geometry, could be extracted from rigorous analysis of ultraviolet charge transfer spectra. Pursuing another aspect of cooperative interactions in inorganic and metal-organic solids, transparent ferromagnets (a class of compound previously quite unknown) were also identified and their remarkable optical properties quantitatively explained by combining advanced optical, magneto-optical and neutron scattering methods. This work pre-dated the recent surge of interest in molecular-based magnets by some years, but gave great stimulus to it. More recently he uncovered totally new types of behaviour in molecular-based magnets, such as giant negative magnetisation and bulk ferrimagnetism involving both pπ- and d-electrons. Mixed valency is strongly implicated in the superconductivity of cuprates and other oxides, and Day's neutron scattering programme not only laid bare the structures of these materials in the superconducting state, but also proved that low frequency vibrations do not furnish the mechanism. Later he brought principles of supramolecular chemistry to bear on molecular metals and super-conductors, revealing unexpected influences on properties, such as molecular conformation, organisation of chiral centres and inclusion of guest molecules.