We usuaiiy climb the Zijin Mountain______ Sunday. (A.on B.in C.at)

on

on
在星期名称前一律用on:
on Sunday/Monday/Tuesday/Wednesday/Thursday/Friday/Saturday

on Sunday morning
on Tuesday afternoon
on Friday evening

A、on,
因为星期前用介词 on.
记住。

A on Sunday 在星期天。这是固定用法，例如，on Monday \ on Tuesday.

A
在星期天

on