> A quick check of Wikipedia suggests Arabic, Portuguese and Vietnamese, all use number-based systems to name the days of the week, and they are indexed from Sunday = 1. But yes, the other is more common. Most of the Slavic languages, and Chinese, among others, use indexed from Monday = 1.
For portuguese the name being number based has nothing to do with when the week starts, portugal is historically a very christian country and always had the week end on sundays, "and on the seventh day he rested", in both casual/formal speech and formally with calendars and such, saturday and sunday are always put at the end.
The names being numerical was because that's how the names were named during Holy Week (the week that ends with easter sunday), christians were supposed to rest and not work during that week, so sunday was the first rest day, monday the second rest day, etc. which is what the names mean, some people today think it means fair as in state fair/market, e.g. monday is "segunda-feira" = "second fair", but it actually means something closer to holiday, the modern portuguese word being férias.
The reason it took over was basically because the church said so, the old names were still based on old gods and the new nomenclature was adopted for all weeks instead of being Holy Week exclusive.
I guess if the aliens landed in portugal that would be another thing to add to the thread :)