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Wikipedia

The names of the days of the week from the Roman period have been both named after the seven planets of classical astronomy and numbered, beginning with Sunday. In Slavic languages, a numbering system was adopted, but beginning with Monday. There was an even older tradition of names in Ancient Indian Astrology which could arguably be the origin of all these naming systems. All of these systems have been adopted in many languages, with some exceptions resulting from a number of religious and secular considerations.