The name origin for each day of the week has its roots in Roman mythology. The days were named after Roman gods and are directly related to names of planets, the sun, and the moon. The modern words used for each weekday in the Romance languages (e.g., Spanish, French, and Italian) are based on the Latin words that correspond to Roman gods (with the exception of Saturday and Sunday). However, while the English words for weekday names have Roman roots, the English names have been subjected to centuries of Germanic and Norse mythology. The Germanic people adapted the Roman system, switching out Roman deity names for German or Norse ones. The following is a list of the origin of the weekday names in the English language with notes on deviations and adaptations of the Romance languages:
Old English Sunnandæg, means literally, the sun's day. English, like most of the Germanic languages, preserves the day's association with the sun. However, the Romance languages have changed its name from associating it with the sun, to associating this day with "the Lord's day" (e.g., in Spanish the word for Sunday is “Domingo” meaning “of the Lord”).
Old English Mōnandæg, means the moon's day. This is similar to the Latin name dies lunae. The Romance languages have preserved Monday’s association with the moon. For example, in Spanish, Monday is called Lunes (the word moon is la luna in Spanish). Both names, Monday and Lunes, are associated with the moon.
Old English Tīwesdæg, means Tiw's day. Tiw is a Norse war god (called Týr), but also the god who, more than any other, presides over matters of law and justice. Tiw also attested prominently in wider Germanic paganism. He seems equivalent to the Roman god Mars. Hence, the name of the day in the Romance languages is related to the Latin name dies Martis, "Day of Mars". In Spanish, for example, Tuesday is called Martes.
Old English Wōdnesdæg, meaning the day of the Germanic god Woden (or Odin in Norse mythology). Woden was a supreme god and was widely known as a god of war. He was also important as a god of learning, of poetry, and of magic. He was a prominent god of the Anglo-Saxons and other Germanic peoples in England until about the seventh century. He was identified with the Roman god Mercury, and among Germanic peoples, Mercury's day became Woden's day (Wednesday). In Spanish, for example, Wednesday is called Miércoles (after Mercury).
Old English Þūnresdæg, meaning Thor's day. Thor is the hammer-wielding Norse god associated with thunder, lightning, storms, oak trees, strength, and the protection of mankind. He is also associated with sanctity and fertility. Thor's day corresponds to Latin dies Iovis, "day of Jupiter" or Jueves, in Spanish for example.
Old English Frīgedæg, meaning the day of the Anglo-Saxon goddess Fríge (or the Norse goddess Frigg). Fríge was the goddess of love, marriage, and destiny. She was a sky goddess, responsible for weaving the clouds, sunshine, and rain which ultimately determined the fertility of the crops. She was also responsible for weavin fates. The Norse name for the planet Venus was Friggjarstjarna, 'Frigg's star'. It is based on the Latin dies Veneris, "Day of Venus". In the Romance languages, Friday is known as Viernes, Vendredi¸ and Venerdì in Spanish, French, and Italian respectively.
Named after the Roman god Saturn, it is the only day of the week that has retained its Roman origin in English. Its original Anglo-Saxon description was Sæturnesdæg. In Latin, it was dies Saturni, "Day of Saturn". Saturn is the Roman and Italic god of agriculture. Saturn ruled in a happy and innocent age, where he taught his people agriculture and other peaceful arts. In the Romance languages, Saturday is not associated with the Roman god; rather Saturday is associated with the day of the Sabbath. For example, Saturday is referred to as Sábado, Samedi, and Sabato in Spanish, French, and Italian respectively.