Yes, you're quite right. The correct form is 'On Monday night'. This is because days of the week are preceded by 'on'. We say 'on Monday night/evening/morning/afternoon' in just the same way as we say 'on Monday'. In fact, the only error in your post was your question. 'I'm right?' is not a question - it's a statement with a question mark after it. Can you suggest some ways of turning 'I'm right' into a correct English question?