Most professional schoolsl require some kind of specalized admissions test. The content and dates of the test vary from discipline to discipline. The sites below include schedules for testing dates, FAQ about the test, and practice tests. The fees for these tests can be expensive so you want to plan out when you take them well in advance and study for them ahead of time.
Normally each year there are two periods to take the MCAT: between January and May an between June and September. Check with the medical schools you are interested in applying to for how far in advance they want to receive your scores. Note that if you are taking the computer verision of the MCAT that you must register for that at least 120 days before the exam.
This is a computer based test offered year round at ProMetric testing centers. You should consult the admissions guide for the schools you are interested in to decide when to take the test based on when they want to receive your scores.