KABUL, Afghanistan — A suicide bomber blew himself up in eastern Afghanistan on Saturday as mostly Taliban fighters gathered to celebrate a three-day cease fire marking the Islamic holiday of Eid al-Fitr, killing 21 people and wounding another 41, said the Nangarhar provincial Police Chief Ghulam Sanayee Stanikzai. Most of the dead and wounded were believed to be Taliban, he said. The devastating explosion came as previously unthinkable scenes of unarmed Taliban fighters celebrating Eid, often alongside Afghan security forces, played out in cities throughout the war-shattered country on Friday and again on Saturday. Within hours of the explosion President Ashraf Ghani announced he would extend a nine-day cease-fire that was to expire on Sunday and which he had unilaterally announced last week.