A turning point in Ireland's war for independence from British rule occurred on this date in 1920. First, a closely coordinated series of assassinations carried out by members of the (old) Irish Republican Army under the direction of Michael Collins crippled British intelligence in Ireland. Later in the day, British Auxiliaries opened fire on players and spectators at a Gaelic football match in Dublin's Croke Park. The massacre of innocents solidified support in Ireland for the Republican cause and prompted international condemnation of the British.
After another eight months of escalating violence during which the British and Irish Republicans fought to a stalemate, a truce was reached in July 1921. Soon thereafter, the Irish Free State was established under the terms of the Anglo-Irish Treaty of 1921.
(Another "Bloody Sunday" took place on January 30, 1972, when British paratroopers fired on marchers in Derry. It was this incident rather than the 1920 events that inspired the U2 song, "Sunday Bloody Sunday.")