Rokuyo was originally used as a unit for distinguishing each day by stars, where one month (calendar) (≒30 days) was divided into five equal parts and six days were regarded as a cycle (30÷5= 6).
六曜は元々は、1箇月(暦)（≒30日）を5等分して6日を一定の周期とし（30÷56）、それぞれの日を星毎に区別する為の単位として使われた。