We are planning a tent camping and hiking trip to the National Parks in Utah. We will have 10 days and wonder if it is best in April/ May or better in September/October.

Which Park/Parks? Bryce tends to be cooler. I've been Mid-October 2 times and Memorial Day weekend 3 times. Mid October has been nice and good fall foliage in Zion and Cap Reef at those times.

Don't miss Goblin Valley. Their campground has showers too.

Not sure there is much difference in weather/temperature between May/Sept or April/Oct. Some of the parks (Zion, Bryce, Arches) may have lower visitation in Apr/May than in Sept/Oct. In Spring, you'll have more wildflowers; in Fall, more colorful foliage. There may also be a higher chance for rain/storms in fall during monsoon season.

Our work schedules make it impossible to take vacation in the spring so we've mostly been out there in late August - late Sept. We experienced really hot temps during our trip last year in early Sept. I'd suggest mid-late Sept just based on our past trips.

We did all the Utah parks from October 1-21. Had gorgeous weather. The colors were beautiful during our visit also.

April/May have generally longer days, but may be windy. Less likely to encounter snakes.

September usually warm and less likely to have snow in places like Bryce or Cedar Breaks.

You can of course get a rainy spell at any time.