It is not proper grammar. However, it does seem to be a popular phrase in the south, since I live in South Carolina and hear it a lot. I teach ELA and have for years. I explain it this way: You cannot do something ON tomorrow. How are you going to do that? Stand on tomorrow, or a piece of paper that has the word tomorrow on it? It is always best to leave that out. EX: We have a test tomorrow. NOT: We have a test on tomorrow.