Day received awards from both the Royal Society (including the Bakerian Lecture in 1999) and the Royal Society of Chemistry; the latter named one of its awards in materials chemistry after him. He  served many national and international agencies and institutions, both professional and governmental, and held numerous honorary degrees and fellowships.
Day was elected a Fellow of the Royal Society (FRS) in 1986, and held the post of Director of the Royal Institution from 1991 to 1998. Day was also director of the Royal Institution's Davy-Faraday Research Laboratory and the Fullerian Professor of Chemistry.
Day was made an honorary fellow of Wadham College, Oxford in 2003 and was a Fellow of the Institute of Physics.