The Whitsundays are the winter cruising grounds for a lot of Sydney yachts so the winter months are when it is all happening. Therefore that is the high(more pricey) season.
This is the time of the SE Trade Winds. So if you picked any time between April and September inclusive you are fairly assured of getting SE winds varying from10-20 knots depending on the position and intensity of the current southern high pressure system.
The next consideration is price. If you look at the bare boat catalogues choose a time in the "shoulder season" so that you are not paying top dollar.
If you time your charter to end on a Saturday or Sunday, they may, on the second last day offer you an extra day or two for free over the radio to save having staff working on the weekend. If you take the boat for 10 days instead of 7 with return of the boat on a Saturday, they might even throw in an extra 4 days.
We were offered an extra day by radio on our second last day but we declined as we had done a big week in our 7 days and had partied pretty hard so we were not prepared for it. If we had been mentally prepared for it we would have taken it. This is likely to happen in the shoulder season as the boat might not be booked to go out for another week.
If you are driving up there it is a good idea not to take the full provisioning package. Maybe three quarter is enough. Then you can do a good shop in Mackay the day before where produce is high quality (especially local meat and veges) and you will save a few dollars or more.
The provisioning package we had was top quality and generous but we did the above and had a bit of food left over.
Another good idea is to take along some extra outboard motor fuel. We nearly ran out and it is expensive and a hassle to get when you are out at the islands.
One place not to miss is Whitehaven Beach and Hill Inlet, particularly if you are taking windsurfers along. Whitehaven Beach has a great BBQ area set up by National Parks but watch out for the Goannas. We left the BBQ unattended and a Goanna took a steak right off the BBQ plate. This was back in 1987 but I have heard they have become quite aggressive since then. A bit like the Dingos on Fraser Island.
Bon Voyage Cisco