Day was born at Kiltallagh, County Kerry, to J. Day, rector of Kiltallagh, and his wife Arabella, daughter of Sir William Godfrey. He was educated at Clonmel Endowed School and Trinity College, Dublin and ordained in 1840. He was the incumbent of St Matthias, Dublin from 1843 to 1868 when he became Dean of Limerick. He was Bishop of Cashel, Emly, Waterford and Lismore from 1872 until his retirement in 1899. He died on 13 December 1904.
- Handbook of British Chronology by Fryde, E. B;. Greenway, D.E; Porter, S; Roy, I: Cambridge, CUP, 1996, ISBN 0-521-56350-X, 9780521563505
- Exact date of birth is noted on his memorial plaque in Christ Church Cathedral, Waterford.
- "Obituary The Rt Rev Maurice Day". The Times (37578). 15 December 1904. p. 5, col F.
- "Alumni Dublinenses : a register of the students, graduates, professors and provosts of Trinity College in the University of Dublin (1593–1860George Dames Burtchaell/Thomas Ulick Sadleir p218: Dublin, Alex Thom and Co, 1935
- "Who was Who" 1897-1990 London, A & C Black, 1991 ISBN 0-7136-3457-X
- A New History of Ireland by Theodore William Moody, F. X. Martin, Francis John Byrne, Art Cosgrove: Oxford, Oxford University Press, 1976 ISBN 0-19-821745-5
- The Times, Friday, Mar 22, 1872; pg. 11; Issue 27331; col D News in Brief
- "The Clergy List, Clerical Guide and Ecclesiastical Directory" London, John Phillips, 1900