Looking on the internet deeply has found these results:

on the first day is the most popular phrase on the web.

in the first day 10 results on the web	on the first day 1147,000,000 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.	(1) A 750 mg loading dose is recommended on the first day of treatment with levetiracetam. I can't believe I'm having a relationship conversation with my lawyer on the first day of my trial. This dose can be started on the first day of treatment. No one makes friends on the first day. Paul, we moved into separate rooms on the first day. You said that on the first day too. It takes some guy six years to get to Team Leader, and another becomes GM on the first day. Failing on the first day I introduce you.

in the first day

10 results on the web

on the first day

1147,000,000 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.

(1) A 750 mg loading dose is recommended on the first day of treatment with levetiracetam.
I can't believe I'm having a relationship conversation with my lawyer on the first day of my trial.
This dose can be started on the first day of treatment.
No one makes friends on the first day.
Paul, we moved into separate rooms on the first day.
You said that on the first day too.
It takes some guy six years to get to Team Leader, and another becomes GM on the first day.
Failing on the first day I introduce you.