Looking on the internet deeply has found these results:

On Saturday morning is the most popular phrase on the web.

In saturday morning 1389,000 results on the web	On Saturday morning 131,400,000 results on the web More popular!
Some examples and use cases from the internet:	Some examples and use cases from the internet:
It doesn't look like this phrase is very popular! The search could not find any examples on the internet.	Office of the European Council's disappointment was plain for us all to see, and this disappointment we shared with him. Just in time for my bat Mitzvah on Saturday morning. in On Saturday morning, the President Auster brothers robbed it on Saturday morning. That particular one was reported stolen on Saturday morning. On Saturday morning, your father and I are having a ceremony. On Saturday morning I had ballet class and Mummy and I went shopping.

In saturday morning

1389,000 results on the web

On Saturday morning

131,400,000 results on the web

More popular!

It doesn't look like this phrase is very popular! The search could not find any examples on the internet.

Office of the European Council's disappointment was plain for us all to see, and this disappointment we shared with him.
Just in time for my bat Mitzvah on Saturday morning.
in
On Saturday morning, the President
Auster brothers robbed it on Saturday morning.
That particular one was reported stolen on Saturday morning.
On Saturday morning, your father and I are having a ceremony.
On Saturday morning I had ballet class and Mummy and I went shopping.