Today is 11-20-09

I heard if I date a check for 11-21-09, the bank won't process it until after that date. Is this true and can anyone explain this?

Actually, checks are processed mostly by machine. So its unlikely that the future date would be noticed.

If you deposit a single check with a teller, they might notice the date and reject it. But if you deposit a bunch of checks or use an ATM, the person prepping the checks is not likely to look at the dates. They just group the checks into same size stacks and then put them in a machine that reads the MICR encoded information.

Yes, most likely it will be just ran though regardless of the date. So you can expect it to go though, if the person you are writing it to knows you want it held, and agrees to hold it, then they will not take it to the bank