The Saint Patrick's Day Four refers to

"four American peace activists of Irish Catholic heritage who poured their own blood on the walls, posters, windows, and a US flag at a military recruiting center to protest the United States' impending invasion of Iraq. The four consisted of a Vietnam War Veteran, former Binghamton City Mayor John Burns' son, and two sisters. Peter De Mott, Daniel Burns, and Teresa and Clare Grady each are members of the Ithaca Catholic Worker community, which teaches that Christians should practice non-violence and devote their lives to service of others. They each served between four and six months in federal prison for their action on Saint Patrick's Day, March 17, 2003, in Lansing, New York, near Ithaca where they reside." 1

Footnotes

1. Wikipedia Article