Days of the Week

The Romans had eight day “weeks,” with the days indicated by letter names (A-H). Each “ninth” day (inclusive counting) was a market day.
The names of our seven day week are post-classical and were based on the names of astrological bodies (with the planets named for divinities). For more on the Latin names, see Isidore. Our English names are based on these Latin names, with the genitive (possessive form: -es) of equivalent Celtic and/or Norse deity names substituted.

Latin French Spanish Italian Rumanian English Names
Dies solis/ dies dominicus (sun/ Lord’s day) dimanche domingo domenica Duminica Sun-day
lunae dies (moon) lundi lunes lunedi Luni Monday (moon)
Martis dies (Mars) mardi martes martedi Marti Tuesday (Tiw: Teutonic deity)
Mercurii dies (Mercury) mercredi miércoles mercoledi Miercuri Wednesday (Woden= Norse Odin)
Iovis dies (Iuppiter, Iovis) jeudi jueves giovedi Joi Thursday (genitive of Thunor “Thunder [god]” cf. Thor)
Veneris dies (Venus) vendredi viernes venerdi Vineri Friday (Norse Frigg, wife of Odin)
Saturni dies (Saturn) or sabbata (Sabbath,) samedi sábado sabato Sâmbata Saturday (Saturn)

Portuguese uses numbers for most of the days: segunda-feira (Monday), terça-feira, quarta-feira, quinta-feira, sexta-feira, sábado, domingo.