From that day on / From that day I knew he's not the guy I should look up to.

[Ashraful Haque]

"From that day (on) I knew he's not the guy I should look up to."

I wonder how dropping the 'on' would change the meaning of the sentence?
Would a native speaker use 'on' here at all?

 

[SoothingDave]

Yes, it's natural with "on." Even though you can omit it and there is no change in meaning.

 

[emsr2d2]

I'd consider "From that day on" (or even "From that day forth"!) almost a set phrase. It's grammatical without "on" but it sounds weird to me.