High Quality Content by WIKIPEDIA articles! The Preparedness Day Bombing was a bombing in San Francisco, California on July 22, 1916, when the city held a parade in honor of Preparedness Day, in anticipation of the United States` imminent entry into World War I. During the parade a suitcase bomb was detonated, killing ten and wounding forty in the worst such attack in San Francisco`s history. Two labor leaders, Thomas Mooney and Warren Billings, were convicted in separate trials and sentenced to be hanged. Rena Mooney and Israel Weinberg were acquitted.