Can someone tell me which is correct. I ate out last Monday. or I ate out on Monday. When I am talking about last Monday- February 12 and today is 18th.
If one says "I ate out on Monday" it necessarily implies the nearest previous Monday.
"Last Monday" generally implies "Monday of last week". For example, if today were Tuesday or Wednesday and someone said "I ate out last Monday", almost no one would wonder if they meant yesterday. But as days pass, particularly towards the end of the week and especially on the weekend, that difference becomes more muddled.