In this phrase the sense of the noun “day” is not to be restricted to a 24-hour period. Note Isa 34:8, which pairs Yahweh’s “day of vengeance” with “the year of recompense” (cf. Isa 61:2), and Ezek 30:3, which qualifies Yahweh’s “day” as “a time of (doom for) nations.” The stress lies more on its character and the event that gives it significance than on its chronological duration. The phrase refers to the time when Yahweh manifests himself and intervenes in the world in a definitive and public way.