Unfortunately, there are many "Bloody Sunday" massacres in the literature:
1. Bloody Sunday (Irish: Domhnach na Fola)—sometimes called the Bogside Massacre—was an incident on
30 January 1972 in the Bogside area of Derry, Northern Ireland, in which 26 unarmed civil-rights protesters and bystanders were shot by soldiers of the British Army. Thirteen males, seven of whom were teenagers, died immediately or soon after, while the death of another man four-and-a-half months later was attributed to the injuries he received on that day. Two protesters were also injured when they were run down by army vehicles. Five of those wounded were shot in the back. The incident occurred during a Northern Ireland Civil Rights Association march; the soldiers involved were members of the First Battalion of the Parachute Regiment (1 Para).
2. Bloody Sunday (Russian: ?????´??? ????????´???, IPA: [kr?'vav?j? v?skr??'s?en?j?]) was the name that came to be given to the events of
22 January [O.S. 9 January] 1905 in St Petersburg, Russia, where unarmed demonstrators marching to present a petition to Tsar Nicholas II were fired upon by soldiers of the Imperial Guard when approaching the city center and the Winter Palace from several gathering points. The shooting did not occur in the Palace Square. Bloody Sunday was an event with grave consequences for the Tsarist regime, as the disregard for ordinary people shown by the reaction of the authorities undermined support for the state. The events which occurred on this Sunday have been assessed by historians, including Lionel Kochan in his book Russia in Revolution 1890-1918, to be one of the key events which led to the Russian Revolution of 1917.
3. Bloody Sunday (Irish: Domhnach na Fola) was a day of violence in Dublin on
21 November 1920, during the Irish War of Independence. In total, 31 people were killed – fourteen British, fourteen Irish civilians and three republican prisoners.
The day began with an Irish Republican Army (IRA) operation organised by Michael Collins to assassinate the Cairo Gang, a team of undercover agents working and living in Dublin. Twelve were British Army officers, one a member of the Royal Irish Constabulary and the last a civilian informant.
Later that afternoon the Royal Irish Constabulary (RIC) opened fire on the crowd at a Gaelic football match in Croke Park, killing fourteen civilians. That evening, three IRA suspects in Dublin Castle were beaten and killed by their captors, allegedly while trying to escape.
Overall, while its events cost relatively few lives, Bloody Sunday was considered a great victory for the IRA, as Collins's operation severely damaged enemy intelligence (especially important since the Irish rebels were fighting a guerrilla war against them) while the later reprisals did no real damage to the guerrillas but increased support for the IRA at home and abroad.
Without knowing which country you are referring to, and what period of history, there's not much I can do to help.
All of the above citations come from wikipedia.org.