The Council Of Piacenza Asks Pope Urban II For Aid Against The Turks
The Crusades had deeply-rooted causes in the social and political situation in 11th centruy Europe.
However, the event which actually triggered the First Crusade was a request for assistance from Byzantine emperor Alexios I Komnenos. Alexios was worried about the advances of the Seljuks, who had reached as far west as Nicaea, not far from Constantinople. In March of 1095, Alexios sent envoys to the Council of Piacenza to ask Pope Urban II for aid against the Turks. Urban responded favourably, perhaps hoping to heal the Great Schism of forty years earlier, and re-unite the Church under papal primacy by helping the Eastern churches in their time of need.
The Council of Clermont was a mixed synod of ecclesiastics and laymen of the Catholic Church, which was held from November 18 to November 28, 1095 at Clermont, France. Pope Urban II's speech on November 27 was the starting point of the First Crusade.
Wikipedia: First Crusade
Wikipedia: Council Of Clermont