There were two Bloody Sundays in Ireland. The first happened in 1920, when an IRA group murdered some British agents because they were ordered to by Michael Colins. The British were pissed, so, they decided to get their revenge by starting a shoot-out at a football match. They killed about 15 Irish Civilians, and in the prisons elsewhere, a couple of IRA members were beaten to death.
The next happened in 1972, when Ireland was split in two, North was held by the British, and south by the Irish (like it is today) and the Irish Catholics in the North were discriminated very heavily, both in public, and in parliament. So, they staged a civil rights march through Derry, which annoyed the British, who murdered a majority of these protestors, all unarmed. This caused a bit of controversey which not a lot of people are happy about to this day.
Guy 1: Bloody Sunday sure wasn't very cool.
Guy 2: Which one?
Guy 1: Both.