Would you experts recommend late May, September or October?
All are great. Probably still some snow on the hoodoos in Bryce and Cedar Breaks (even higher than Bryce)in may and it's wildflowerseason but october is foliage season. Yes I vote for end of september/begin of october. Less bugs too. Like to know what my fellowreviewers think.
Another vote for September/October. The weather should still be warm, maybe a little cooler at Bryce Canyon because of the altitude, and you may start to see the Fall colours. You'll certainly find the Ranger Programmes still operating then. We visited Bryce Canyon in May 2005 and drove through a snowstorm, and found some of the roads we wanted to travel still blocked by snow, but that's Mother Nature for you.
Tony, another poster said: "Only fools and newcomers try to predict weather" I tell you because we visited Bryce and Cedar Breaks (first opening that day!) both mid may and end of september (not in the same year.) Gorgeous weather, blue skies, shorts and short sleeves and 75F both times and snow in Bryce mid may was gone (still lots of snow in Cedar Breaks, not on the roads though). You really can't predict it....(september 1995 and may 2001, it's about time again but there are so many other gorgeous states too..)
I was at Zion and Cedar Breaks in early October of 2005. Zion was very comfortable, as temps were probably in the mid-70s during the day. We got to Cedar Breaks about 30 minutes before sunset, and temperatures there were in the upper 40s. So while Cedar Breaks was a bit chilly, being able to experience Zion with comfortable temperatures was well worth it.
For the record, I was also at Zion in June of 2004. Temperatures were in the upper 90s, so it made for some very warm hikes and some warm evenings to camp.
End of September is my vote. Best chance of catching the aspens as they're changing color.
As several other posters have suggested, September or October would be an ideal time for your trip. Just keep in mind that, even though Bryce and Zion are only an hour and a half drive from one another, the difference in elevation is huge. Bryce is often the coldest place in Utah and Zion one of the hottest. So you'll want to find a happy medium. I'd personally not risk going to Bryce after about the middle of October. The weather could be gorgeous then, but you never know. It is, after all, over 8000 ft above sea level.
Definately late April! I went this past April 19th - 23rd (for my 35th birthday) and the weather was perfect. The most amazing blue sky, about 78-80 during the day and about 58-65 degrees at night. It never rained and it wasn't crowded at all. We had no problem getting on the shuttle except for the saturday we were there which was also earth day (my birthday) it was very crowded and so we got a taste of what it must be like during the summer's peak season. In April we got to comfortably hike, rent bikes, see waterfalls and really enjoy the park. I've also heard that the fall is beautiful. No matter when you go Zion is an amazing place. around every corner a new more beautiful view opens up. It's impossible to take a bad picture and just being there is very calming. Take the time to rent bikes if you can, it's a nice intimate way to see the park. You can pull over and see dear upclose or walk over to the river when ever you want; you aren't at the mercy of only stopping at shuttle stops, and you get to see things you would have otherwise missed. Have a great trip!
Cedar Break had not opened, I think due to a scheduled Memorial Day opening, not to snow.
Sure there are people already in these places, but not that many and very few kids. In fact the Zion buses were only half full.
We really liked the moderate hikes in Zion and Bryce, especially, in the latter, descending into the Canyon for a very different perspective.
We met folks in Moab who were in Bryce a few days after we left, and it snowed on them.
Cannot depend on the weather.
Enjoy, it is fantastic country.