。
The day we agreed on is coming up.
Literal
Promise [gen-の] day [subj-が] is-approaching.
約束の日 — '(the) promised day / the day we agreed on.' The の here is the general noun-linking particle, not possessive. 迫っている ('is pressing close, is impending') is the ~ている form of 迫る, a verb that inherently marks approach toward a temporal or spatial limit. Works for deadlines, approaching dangers, and encroaching events.