You can take the test whenever you want.. Im just saying to see if your body is REALLY 100% clean i'd do it during your first void of the day.. just by drinking however much water you could be diluting your urine and passing without even knowing... My reasoning for this is simple.. What if you took the drug test today after drinking a few bottles of water and passed it.. then you went in for a drug test tomorrow morning using your first void of the day (potentially the dirtiest) without drinking water and you fail?