the date of the introduction of this system is not known exactly, but it must have happened later than ad 200 but before the introduction of christianity during the 6th to 7th centuries, i.e., during the final phase or soon after the collapse of the western roman empire. this period is later than the common germanic stage, but still during the phase of undifferentiated west germanic. the names of the days of the week in north germanic languages were not calqued from latin directly, but taken from the west germanic names.