Latin names for the days of the week are quite like Sanskrit names. Starting with Sunday they are named after the Sun (Ravi or Āditya रवि or आदित्य), the Moon (Soma or Candra, सोम or चन्द्र), Mars (Mangala or Bhauma or Kuja, मङ्गल or भौम or कुज), Mercury (Budha बुध), Jupiter (Bṛhaspati or Guru, बृहस्पति or गुरु), Venus (Śukra, शुक्र), and Saturn (Śani, शनि). In short, the order of the planets for the day names is:
Sun — Moon — Mars — Mercury — Jupiter — Venus — Saturn
This is not the same as their order in the solar system so how was the order of the planets chosen? And why are these names the same across many cultures but not in all? For example, in Greek, Arabic, Hebrew and Slavic languages they are ordered as Day One, Day Two, and so on.
In languages such as Germanic and Persian, the names of deities do not appear at first sight to correspond to the planets in the list.
In Germanic languages, Tuesday through Friday are named after the deities Týr, Odin, Thor, and Freya, from where we get Tuesday, Wednesday, Thursday, and Friday.
In Old Norse sources, Týr is described as the son of Odin, who is Mercury (Budha in astrology is Viṣṇu). In other words, Mars (Skanda, स्कन्द, general of the devas) is viewed as son of Viṣṇu, rather than of Śiva (this variant story may be seen in the context of Skanda Upaniṣad, which tells us that Viṣṇu and Śiva are identical).
Thor in Germanic mythology is a hammer-wielding god associated with thunder, lightning, storms, oak trees, strength, the protection of mankind, and also hallowing and fertility. The Romans saw Thor as the German analog of Jupiter, and his hammer was viewed as similar to the club of Hercules. The hammer of Thor is also similar to the plough (hala) used as a weapon by Balarāma.
The Indo-European linguist Émile Benveniste saw Freya or Frigg as cognate with Priya, a daughter of Dakṣa, and thus a generic name of the Goddess.
In Persian, the names of the days (ruz) starting Sunday are Mehr ruz, Māh ruz, Bahrām ruz, Tir ruz, Hormazd ruz, Nāhid ruz, and Keyvān ruz. A little bit of analysis shows that they are not really different from the Indian scheme. Mehr is Mitra, another name for Sun; Māh is Māsa which is Moon, Bahrām is derived from the earlier Verethragna that corresponds to the Sanskrit Vṛtrahana which means Mars (slayer of Vṛtra who was originally Indra but in later stories was identified with Skanda).
Tīr तीर, arrow, hearkens to the archer Arash’s arrow that covers part of the globe which story is a retelling of the three steps of Viṣṇu. Some scholars see Arash, the archer Ǝrəxša (Yasht 8.6), as similar to Rudra, but I think in the identification with Mercury, the story of Viṣṇu holds more weight.
Hormazd is Ahura Mazda, the prophet-teacher of the Persians, fulfilling the role of Guru, Nāhid is from the goddess Anāhitā associated with water, fertility, wisdom, warfare, and eventually the planet Venus, and Keyvān is Saturn. Some see Keyvān borrowed from Mesopotamia but that is not necessary since it means the greatest poet-sacrificer (kavi) for it was known that this planet was furthest from Earth.
Thus the evidence suggests that the names of the days were originally after deities, but after the triumphs of Christianity and Islam, certain cultures were compelled to abandon the system that to the new rulers smacked of idolatry.
The Slavic nations were victims of the Northern Crusades and they were violently separated from their old tradition. In contrast, the Germanic nations, which were Christianized more slowly, held on to the names of their deities although they also lost the deeper connection. The growth of Christianity in Southern Europe was even more gradual and the earlier names held on.
Greek Names for the Days
It is most interesting that the Greek names for the days follow the numerical system and are different from the Indian names: Κυριακή (Kyriakí), Δευτέρα (Deftéra), Τρίτη (Tríti), Τετάρτη (Tetárti),Πέμπτη (Pémpti), Παρασκευή (Paraskeví ), and Σάββατο (Sávato). Thus Sabbath (Savato) is the seventh day.
There is a theory that the names for the days of the week came from Greek astrology and spread across Eurasia in the first or second century CE. But this theory is clearly wrong for it cannot explain how Rome and India across the borders of the Greek world on opposite sides have the same system, different from the Greeks.
We know that the Indians had much to say about the planets in the third millennium BCE, long before the Greeks or the Babylonians, and the sparsely populated Śaka corridor served as a passage for cultural diffusion from India to the Slavic world and beyond. India and Rome also had much sea trade during the imperial period and that could have served as an additional conduit for the diffusion of ideas.
Order of the Planet Names
The Moon, Mercury, and Venus are the inner planets, whereas Mars, Jupiter, and Saturn are the outer ones. The sidereal periods (referenced to the distant stars) are proportional to the distance from the Sun and the closer a planet is to the Sun the faster it moves. The planet names for weekdays alternate between the outer and the inner planets.The order of the planets starting with the Sun which takes place of Earth and after the Moon going out to Saturn and then down is:
Sun — Venus — Mercury — Moon — Saturn — Jupiter — Mars
In this scheme, the two referents are the Sun with which we begin the order of the inner planets and Saturn with which we begin the order of the outer planets. According to the Padma Purāṇa, Saturn is the son of the Sun, which appears to support this assignation. (Additionally, in the Sūrya Siddhānta, Mars is born to Earth, and Mercury to Moon hence also called Saumya.)
An interesting astrological theory regarding the order chosen is desscribed in the Sūrya Siddhānta. The first hour of the first day of the week was ascribed to the Sun in the series related to distance from the Earth.
Saturn — Jupiter — Mars — Sun— Venus — Mercury — Moon
The hours were thus marked to Sun, Venus, Mercury, Moon, Saturn, Jupiter, Mars. The eighth hour of the first day was ascribed again to the Sun, and also the fifteenth and twenty second; the twenty fifth hour (the first hour of the next day) is the Moon, a shift to the right of three, which mathematically is the equation 24 n modulo 7, where n is the day count where the start is n = 0. Following this cycle for every hour and every day in the week, the first hours of the following days after Sunday will be ascribed to Moon, Mars, Mercury, Jupiter and Venus, which are the familiar names of the days.
This system also assigns the planets to months and years.
It is not surprising that modular mathematics was at the basis of the astronomy of altar ritual of the second millennium BCE.
After the rise of Christianity an attempt was made to switch the order to make Sunday the day of rest or the last day of the sequence.