Which One Should Be Used?
Both 'on month' and 'in month' are correct. 'On month' is used to refer to a specific day in a month. 'In month' is correct when referring to the span of one month.
When we want to talk about an action or event that occurred on a specific day or date of the month, we use 'on month' followed by the exact date. For example:
I met him
We use the preposition 'in' to talk about long periods of time such as years, centuries, and months. 'In month' is a more general phrase than the phrase 'on month.'
The documents were collected
The yearly company meeting is