We always have a party the evening 31st October.Ain; inBat; inCon; ofDon; on

问题补充：

We always have a party the evening 31st October.

Ain; in

Bat; in

Con; of

Don; on

答案：

A. in; in

B. at; in

(答案→)C. on; of

D. on; on

解析：句意:我们总是在10月31日开晚会。在具体的某一天上午，下午或晚上应该用介词on；用介词of表示“……的”，根据句意故选C。