The names of the 7 days of the week were inspired, and originated, from the Sun, Moon, and the five planets that were visible from the naked eye in the Greek civilization. The Greeks were the first to assign 24 hours to a single day, and astrologers believed that each day was ruled by one visible celestial body, with the first being the Sun, followed by the Moon, then Mars, Mercury, Jupiter, Venus, Saturn. The Greeks referred to the day names as "theon hemerai" meaning "Days of the Gods".
Roman culture adopted the 7 day week as well, but the week day names were changed to correspond with the Roman Gods. The week day names were eventually adopted by the Germanic people. The Greek and Roman cultures were both known to the Germans resulting in day names being influenced by both cultures.