Sunday July 10, 1921. Bloody Sunday.

This day became known as "Bloody Sunday" in the Anglo Irish War, but its just one of several days termed that in association with Irish conflicts

Tensions were high leading into the agreed to July 11 cease fire between the UK and the leadership of Dáil Eireann, the putative legislative body of Ireland, with some protestants in Northern Ireland feeling they'd been sold out as the British clearly intended to come to an agreement resulting in Irish independence.  In the early morning hours of the 10th the Royal Irish Constabulary attempted to launch a raid on Republican sites in advance of the truce, but the Irish tactic of blowing whistles and banging on things, used for decades, frustrated the attempt and the IRA ambushed a RIC truck.  General violence broke out in the city thereafter.

Elsewhere, and unconnected with Belfast, a group of IRA men caught and executed four British soldiers in what seems to have been a reprisal for an earlier British killing of an IRA man.

The events perhaps indicated that while arriving on a negotiated treaty giving Ireland independence was now inevitable, in some quarters of the country some level of violence might occur in the future.

Also, like a lot of events of the Anglo Irish War, while the occurrences were really horrific, the level of death that's associated with them by myth isn't as high as it was in reality, which none the less doesn't reduce its tragic nature.