Καὶ ἐγένετο ἐν ἐκείναις ταῖς ἡμέραις
And it came to pass in those days. Mark 1:9a
This is more of a Jewish transitional clause than an idiom. It was a way to introduce a new subject or event. In the greek, the passage literally says, “and it became in those days.” I think “and it came to pass” is a wonderful english translation of this transitional clause.