Christmas was commissioned through NROTC as a Second Lieutenant in the U.S. Marine Corps Reserve in 1962. He received his diploma at the University of Pennsylvania with a Bachelor of Arts degree. After completion of The Basic School at MCB Quantico in Virginia, he was assigned as a platoon leader, 3rd Battalion, 2nd Marines 2nd Marine Division, Camp Lejeune, North Carolina. In May 1965, he was assigned to the Marine Barracks in Washington, D.C. where he served as the Executive Officer. Upon promotion to captain in June 1966, he served as Commanding Officer, Headquarters and Service Company. He transferred to South Vietnam in July 1967, where he served successively as Commanding Officer, Service Company, Headquarters Battalion, and Commanding Officer, Company H, 2nd Battalion, 5th Marines, 1st Marine Division, FMF.
During the Battle for Hue City in 1968, Christmas was seriously wounded and evacuated to the Philadelphia Naval Hospital. For his actions of “extraordinary heroism” in Hue city on February 5, 1968, he was awarded the Navy Cross. AFter recovering from his wounds, in October 1968, he was assigned to the staff of The Basic School in Quantic, Virginia; he subsequently attended the Amphibious Warfare School, graduating with distinction. Christmas returned overseas in July 1975, for duty as the Operations Officer and, later, Executive Officer, 3rd Battalion, 4th Marines, 3rd Marine Division, FMF, in Okinawa, Japan. On July 15, 1994, he assumed his final active duty Marine Corps post as Deputy Chief of Staff for Manpower and Reserve Affairs. He retired in 1996 after 34 years of active duty service.