I'm not sure that one day is going to be enough to flush your system without buying something. The THC sits in your fat, and the more body fat you have the longer it'll stay in your system.
If you know that you couldn't flush your sytem in time-- there are some things you can do at the testing facility that will help you at least by some time. Certain meds can throw false positives which can leave the results as inconclusive or they'll typically ask you to take another. There will be a spot on the form where it asks you to lists medications you are currently taking. Meds for flu and cold(with ephedrine), weight loss pills, prescription meds for anti-anxiety or antibiotics can all throw false positives. I know many vitamin supplements that have B2, can throw a false positive for THC.
This might at least be a way to buy you some time until you can actually flush your system, and give you a legit reason for why you might of failed. Good luck bud.