。
We're meeting at 9 p.m. tomorrow night.
Literal
Tomorrow [possessive-の] night 9 o'clock [at-に] gathering is.
集合です is a classic Japanese 'noun-as-predicate' construction: 集合 (gathering/assembly) is a noun, and です turns it into 'it is [a] gathering.' English has to flip this into a verb ('we're meeting'). The particle に marks the time of the event. Who's gathering is left unstated — the listener knows.