Question

let's make it(/)half past eight(on)the morning of June 21为什么

Answer 1

(on)the morning of June 21 在六月21日那天的上午,因为有具体的日期,所以用介词on

at 用在点钟前,如:at six,或:at night,at noon

in + 年/ 月/ 季节..如:in 2010,in May,in spring