。
If it rains, we'll postpone our departure.
Literal
By-chance rain [subject-が] fell-[conditional-たら] we [topic-は] departure [object-を] will-postpone.
延期する ('to postpone') is the formal verb for pushing back a scheduled event. 我々 ('we') is the formal/literary first-person plural pronoun, more dignified than 私たち. The combination marks this as a written-register sentence despite the conversational shape.