For up-to-date information on the MCAT visit the official Medical College Admission Test site at the American Association of Medical Colleges:
http://www.aamc.org/students/mcat/start.htm
That site includes schedules for testing dates, FAQ about the test, and practice tests. 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.
