What is Blood Sunday?

Answer 1

If you mean bloody Sunday, then it can refer to 2 events in Irish history:

1.Bloody Sunday ,Dublin, 1920.

A day of extreme violence in the Irish war of independence, in the morning of 21st of November the IRA commanded by Michael Collins launched an attack against the British armys intelligence network in the Irish capital of Dublin, 13 british intelligence officers were killed. In retaliation British soldiers and policemen drove into croke park during a Gaelic football match and opened fire on the crowd, shooting over 70 civilians and killing 14. Later in the day the british tortured then executed 2 IRA prisoners.

2.Bloody Sunday ,Derry City, 1972 (more famous)

During a peaceful civil rights and anti-internment protest in the bogside area of Derry City British soldiers from the parachute regiment opened fire on the demonstrators, 29 people were shot and 14 were killed, 2 were shot while lying on the ground , 6 of the dead were teenagers, 1 was killed while waving a white hankerchief to indicate his peaceful intentions and another was shot and killed while he had his hands up and was shouting "Jesus Christ Dont Shoot". All of the dead and wounded were proven to be unarmed despite the soldiers claiming they were fired upon. This turned out to be the most important incident of the Irish 'Troubles' as many people joined or began supporting the IRA in the aftermath of the massacre.