(on\in\at)___Friday morning.

(on\in\at)___Friday morning.
(on\in\at)___Friday morning.

(on\in\at)___Friday morning.
on Friday morning.
例句
It was Tom that I met in the park on Friday morning.
我星期五早上在公园见到的是汤姆.

On Friday morning!
具体某一天的早晨，中午，晚上用On。
而只说一天的早晨，中午，晚上用IN。如in the morning

on

ON Fri. Morning
只要后面加了day就必须用on

on加具体某一天 不管后面是否有具体早中晚