Once you register at the USMLE website, you choose a 3 month period in which you want to take the exam. They'll then send you an orange document with a registration number with which you can go to the prometrics website to choose a location and test date. When you select your date depending on the schedule and availability of the site. I scheduled my test a few days ago, and as I recall, the potential dates for my test was 3 times every weekday (you can schedule your exam to start at 8:30. 10:30, or 12:30), one starting time on saturday, and maybe on sunday (I forget if it's closed on sunday or not).
The thing you probably want to keep in mind is that if you're looking for a may or june test date, the phrase "ASAP" should reverberate in your mind. When you register on the USMLE website for your test, you have to print out a confirmation form thing, where you attach a 2x2 picture and give it to your registrar and then he sends it to the NBME and then they send you a document with a registration number on it that you need to set your test date. There is a delay of a couple of weeks between when you submit the initial application and when you get the registration document, and I'm sure that delay will be even longer over the holiday even if you were to get the confirmation form to your registrar as early as tomorrow.
