The Mediterranean season normally kicks off in April/May, with some yachts crewing up earlier than that. The boats will typically start to head over to the Caribbean from September onwards, many attending the Ft. Lauderdale boat show at the end of October, or the Antigua and show in December.
The seasons are approximately four to five months long. The remaining months are typically used for yard periods and crewing up if need be.
It is difficult to say when exactly seasons start and finish, as each year is different. Hiring periods are generally quite broad, as there are some yachts who crew up early before the season gets going while others take on delivery crew for crossings and then crew up last minute on the other side.