From Wikipedia

The Great War In Colour - Beginnings

The first day on the Somme, 1 July 1916, was the opening day of the Battle of Albert, which was the first phase of the British and French offensive that became known as the Battle of the Somme. The middle day of the middle year of the First World War, it is remembered as the bloodiest day in the history of the British Army when 57,470 men became casualties of which 19,240 were killed or died of wounds. In terms of British casualties, the first day of the Somme is only surpassed by the Fall of Singapore in World War II when over 80,000 Allied soldiers became prisoners of war, the Battle of Watling Street in AD 60 or 61, when 80,000 Britons are said to have died fighting Roman forces, and, as a percentage, by the Battle of Towton in 1461 in the Wars of the Roses, when 1% of the English population died.