My employer already knows the day I plan on starting (assuming I pass the drug test of course) so I can't call and ask that. What would be the next best thing to say to them?

I was thinking of just emailing her (since that is the way 90% of the communication between us has been) and saying:

"Good afternoon,
I took my pre-placement drug test last Friday, so I should be all set to start working on May 20th. Is there any paperwork I need to fill out prior to the start date?"
I would leave out any and all references to the drug test altogether. Just skip on to the inquiry about the "start date" and/or "paper work", if anything.

If a start date has already been established, just simply show up for work and carry on with your duties with that "drug-free person" mindset. And dont discuss anything about the drug test with anyone. You want to be viewed as a "drug-free person", right?