Regardless of the time of day you decide to fly, the ticket price will remain around the same. Book flights from West Palm Beach to Grenada as you normally would.
Due to multiple factors, expect to potentially pay more when booking in December. For low rates, July can be a great choice for travel to Grenada. Flights to Grenada from West Palm Beach in July are about $469 on average during July, but can be found for as low as $268.