The scenic beauty of the Canyon, the many amazing waterfalls, and camping along Havasu Creek is awesome all year around!
Cooler months of the year are ideal for hiking and exploring. Warmer months are ideal for hanging out in the water (but also means planning your hikes in and out for the early morning when it is cooler).
The water temperature is roughly 70 degrees all year long. The air temperature is similar to that of Phoenix - and once you reach the Campground there is lots of shade and water to help keep you cool.
In the end though, since all dates sell out almost instantly, the best plan is to be as flexible as possible and grab whatever dates are still available.