Hello!
Is "on" necessary in this sentence, please?
"I'll see you on another day!"
Thank you
Will
Doesnt appears necessary to me... You can simply say: I'll see you the other day/ or some other day.
I'm not a teacher
I agree with Huda-M's basic answer (you don't need the word "on") but I have corrected some errors in their post, particularly their suggestion that "I'll see you the other day" could be used. It could not be used. "The other day" refers to a point in the past, not the future.
I'll see you another day.
I'll see you some other day.
I'll see you some other time.