hello my friends ,
I would like to know which one is correct
in this day …
or
on this day …
thanks
On this day.
That would generally be correct, but there are some exceptions, such as “In this day and age …”
It is always best to provide full sentences so we can better assess what you mean.
it’s a poem I wrote : " in this very day , I wish all the trees sway … " is that correct ?
You should use “on.”
“In this day” usually refers to an era rather than a specific day.
In this day of struggle, we must unite.
thanks a lot Mordant 
It seems we are always on a day but in x days or an era. Isn’t it because we live on a day, a carrier only carrying us who are awake? Once we sleep, we get in days?