I would only use 1. A comma is optional.
However, 'I was born in April(,) 1991' is correct.

Student or Learner
Hi,
1- I was born on 17 April 1991.
2- I was born in 17 April 1991.
3- I was born 17 April 1991.
Should I use 'on' or 'in' in such a sentence above? I think 'on' sounds OK and also that I should put a comma after 'April.' I am not sure about the third one.
Thanks.
I would only use 1. A comma is optional.
However, 'I was born in April(,) 1991' is correct.
Americans usually write April 17th,
1991. The th is optional and is always omitted in the day-first notation.
I am not a teacher.
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