in the morning/on the morning

☆「in the morning」
I usually have meetings in the morning.

the morningの前につく前置詞は後ろに引っ張られる。

☆「on the morning + of the + 日付(序数)」
I’ll have a meeting on the morning of the 15th.

☆「on the morning + of + 月 + 日付(序数)」
I’ll have a meeting on the morning of June 15th.

☆「on + 曜日 + morning」
I’ll have a meeting on Monday morning.