I would consider the summer over XMAS. I suggest pricing out the two to compare. I believe XMAS prices are higher than summer for airfare, car rentals and hotel /condo rental rates. Less Availability mean higher demand and higher prices.
Also, if you choose a summer visit, you'll have more resorts to choose from if you search across all possible summer dates. Keep in mind, summer is also peak travel time (with the height of demand starting from mid June through mid August for two bedrooms or larger).
Good Luck
The only appeal of Hawaii to me personally is to get somewhere warm in the dead of winter . Our summers are typically too packed to wedge in another vacation. I'd consider Spring Break, but I'd guess availability is pretty sparse at that time as well.