Earlier quoted context omitted.
> 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. I can confirm that in Hebrew, the name for Sunday translates into "First Day", Monday into "Second Day", etc. Except for Saturday, which is Shabat.
Shabbat (apart from meaning "rest") is also generally accepted as deriving from the Hebrew word for "seven", which is "sheva". And Shabbat is sometimes referred to in Hebrew as "yom shvi-i" ("seventh day").
Klein Dictionary gives the etymology of שַׁבָּת as derived from שׁבת (to rest). I also found references suggesting the origin is with the Akkadian šapattum, the 15th of the month, but none related to שׁבע (which originates in Proto-Canaanite).
היּוֹם הַשְּׁבִיעִי (note the definite form!) is how Saturday is referred to in Genesis 2:2-3, without actually having the name שַׁבָּת. There's a some discussion about why שַׁבָּת isn't mentioned there as a name for the day, but regardless, your comment is the first time I read of the etymology of שַׁבָּת deriving from שׁבע.