He was educated at Gresham's School and the University of Oxford, from 1958-1962, where he gained a B.A. in Mathematics and a Diploma in Statistics, and the University of Aberdeen from 1962-1966, where he obtained a Doctorate of Philosophy.^{[citation needed]}

Day worked at the International Agency for Research on Cancer in Lyon from 1969 to 1986, where he rose to become head of the Unit of Biostatistics and Field Studies.^[1] He was director of the Medical Research Council Biostatistics Unit from 1986 to 1989, and continued as honorary director until 1999.^[1] From 1997 until his retirement in 2004 he was co-director of the Strangeways Research Laboratory in Cambridge.^[2] He was also professor of public health at the University of Cambridge from 1989 to 1999, and professor of epidemiology from 1999 until 2004.^[1]

Day was made a Commander of the Order of the British Empire in the 2001 New Year Honours for services to statistics and epidemiology underpinning cancer biology.^[3]

His hobbies include sea fishing and tree growing.^[1]

• 1980 (with Norman Breslow), Statistical Methods in Cancer Research: Volume 1—The Analysis of Case-Control Studies. International Agency for Research on Cancer, Lyon, France. ISBN 978-92-832-0182-3.
• 1986 (with Matti Hakama & Anthony B. Miller) Screening for cancer of the uterine cervix. International Agency for Research on Cancer, Lyon, France. ISBN 978-92-832-1176-1
• 1987 (with Norman Breslow), Statistical Methods in Cancer Research: Volume 2—The Design and Analysis of Cohort Studies. International Agency for Research on Cancer, Lyon, France. ISBN 978-92-832-1182-2.
• 1988 (edited with Anthony B. Miller) Screening for Breast Cancer. Huber. ISBN 978-0-920887-26-4.