In the EOTC, both Saturday and Sunday are observed as holy, but Sunday is treated as primary/the greater of the two.
Regarding Judaism among the Ethiopians predating Christianity, that’s certainly what their origin stories say, but it is not universally accepted among Rabbinic authorities that they are actual Jews. The Beta Israel (Ethiopian Jews) have faced a lot of discrimination in Israel because of this (you can see a dramatization of this 2005 French film “LIve and Become”). Even outside of Jewish law interpreters, Jews in entirely other fields such as ethnomuiscologist Kay Kaufman Shelemay have hypothesized that much of their ritual actually either derives from or developed in tandem with and under the influence of Orthodox Tewahedo church ritual. While they use slightly different instrumentation, the basic melodic structure of their services is very similar to that of the EOTC, and like the EOTC they use Ge’ez as their liturgical language (and speak Amharic or Triginya natively, though traditionally they spoke Agaw/Cushitic languages, and some of their closest ethnocolutural relatives, like the Qemant people, still speak these languages in tiny numbers back in Ethiopia, in Gondar zone of the Amhara region).