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1. on the early/late morning of March 7

2. in the early/late morning of March 7

Which of the above expressions is more idiomatic in American English?
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Hi Sitifan

I would normally say:

- on the morning of March 7
- in the early/late morning of March 7

In other words, the addition of the word "early" or "late" would likely result in my choosing "in" rather than "on".
Comments  
I agree with yankee. 

"on the morning" suggests that the entire morning is referenced.

"in the early/late morning" suggests that a period of time smaller than the entire morning is referenced.