Day participated in the capture of Peleliu and went ashore on September 15, 1944. He was appointed Peleliu Air Base Commander and was responsible for the operation of the airfield, which became the principal staging area for the Philippine Campaign. He was promoted to the rank of colonel during that time and served in this capacity until June 1945, when he assumed command of Marine Air Group 21 on Guam and also held additional duty as Commander of Transport Air Group.