Can I get drunk on saturday night if I'm getting an MRI on my brain on monday afternoon?

Yes but should you?? The answer is no, drinking alcohol, even to excess, should not effect you MRI on monday. But, if you need an mri, then I am guessing you are having some kind of neurological symptom(s). Therefore alcohol should be the last thing you should be putting in your system (and, thus, your brain). Give your brain a rest until after your doctor tells you it's ok to drink again, ok?
Simple answer, NO. If you are asking if drinking too much alcohol will effect your brain, the answer is yes. Whether it will effect the results of an MRI two day after getting drunk would be hard to tell. In any event, i would urge you to take a close look at how much you regularly drink. If you need a reason not to get drunk than perhaps you are drinking too much.