While it's acceptable to call good friends on a Saturday afternoon for a get-together that night, it's definitely not OK to do the same with someone you're dating. Calling for a date at the last minute says, "I'm not interested enough in you to make plans in advance."
Accepting a date at the last minute is equally problematic. It says, "I'm desperate. I have nothing better to do than go out with you this weekend." By agreeing to go, you're setting the tone for a relationship in which you allow yourself to be treated like a second thought. Expect lots of last-minute phone calls.
This brings us to another no-no: cancelling a date at the last minute. If you've suddenly come down with a horrible stomach bug and you can't stop vomiting, or if you're going to be fired unless you work until midnight, you've got an acceptable out. Otherwise, it's not alright to call an hour before a date and say your plans have changed. Unless, that is, you're hoping you'll never see the person again, because that's exactly what might happen.