万一雨が降れば、園遊会は行われないでしょう。

、。

If it happens to rain, the garden party probably won't be held.

Literal

By-chance rain [subject-が] falls-[conditional-ば], garden-party [topic-は] won't-be-held [probably-でしょう].

園遊会 ('garden party') is a formal outdoor gathering — famously associated with the Imperial Garden Party held twice a year by the Emperor. 行う ('to hold [an event], to conduct') is a formal verb for carrying out events. The passive 行われる makes 園遊会 the subject: 'is held.'