We plan our trips for the spring and fall and find the best time times to go are in April, and mid October to mid November. Each season has certain advantages, but both avoid the hottest part of the hiking season in Grand Canyon. It's no fun hiking the inner canyon in the summer. Ask anyone who's done it. It's HOT!. And, it's nearly impossible to carry enough water to make any multiday trip safely, especially if you're relying on water sources other than the Colorado river, which you must to for certain stretches of the Tonto plateau.
April in the Canyon is good hiking weather. It's still cool in the inner
gorge and as importantly, most of the seasonal water supplies are running,a
result of the snow runoff on the rim. On a recent trip over the Tonto from
the South Bass to the Hermit trail we found water in every canyon. Not
having to carry more than a liter or two of water in an already bulging
pack is always welcome.
Fall is beautiful at the canyon, with the deciduous trees in the upper
region changing to their autumn hues. Fall tends to be a little drier and
I'd say mid October is an optimum time to do serious backpacking. There
is less water to be found but it can be found if you look closely and carry
a filter.