The days of the week are named after planets and their associated deities: Sunday (Sun), Monday (Moon), Tuesday (Mars/Tiw), Wednesday (Mercury/Woden), Thursday (Jupiter/Thor), Friday (Venus/Freya), Saturday (Saturn).
This planetary hour system determines which planet "rules" each day. Sunday is ideal for vitality and leadership (Sun), Monday for emotions and intuition (Moon), Tuesday for action and courage (Mars), and so on.
This ancient astrological practice continues in our modern calendar, though most people don't realize they're following a 2,000-year-old astrological system every week.