The Second World War, lasting from the German invasion of Poland in September 1939 to VJ Day in August 1945, only encompassed six Christmases.
Some, such as Steven Weisenburger, author of A Gravity's Rainbow Companion, have written this off as an error on Pynchon's part. However, Pynchon gets it right later when he mentions 'six years of slander, ambition and hysteria.' It is probable that the 'mistake' is an intentional device meant to convey Roger and Jessica's confusion. They are, quite literally, at sixes and sevens.