In ancient times, periodic events were classified such as Moon periods (28 days). The full moon, half moon, dark moon and half moon periods formed seven days and hence a weekly period. Names given to each day of the week were originally ascribed to planets--Latin names still having similar names. There is also a relationship to Norse religion, Teutonic mythology and Astrology. The relationships are:
Sunday (the Sun), Monday (Moon), Tuesday (Mars--Mardi), Wednesday (Mercury--Wodens day), Thursday (Jupiter--Thor), Friday (Venus--Fria) and Saturday (Saturn).