Mark 16:2

2 Et valde mane una sabbatorum, veniunt ad monumentum, orto jam sole.

what does una mean here exactly? It should mean “one” but “first” seems better in context. But una is a cardinal number not ordinal. Is una ever used as an ordinal number like prima?

It just translates the Greek literally (τῇ μιᾷ τῶν σαββάτων), so your question is more about the Greek than the Latin. This formula for day 1 of the sabbath (or of the week) is in line with e.g. Matt. 28:1 (εις μίαν σαββάτων) and Paul too (1 Cor. 16:2 κατὰ μίαν σαββάτου).

una is ablative, I trust you realize.