The word menstruation derives from the word mensis (plural: menses), Latin for month. A lunar month is meant here, not a calendar one.
The average time between two menstrual periods is about the same time as the moon needs to circle the earth. No surprise then that the moon often symbolises something feminine. Already since ancient times it’s frequently associated with fertility and motherhood.
Only one percent of all women have a cycle of exact 28 days; everything between 21 and 35 days is considered normal.