We visited in October outside of the holiday season and summer heat. We also arrived on Wednesday when crowds were lower, on Friday and Saturday the crowds were noticeably higher, but not unbearable.
The temperature in October is also perfect, not too hot or cold during the day and evening. Each morning the wind is up and it was cold so bring a light jacket that you can take off and easily carry as you hike.
Parking in the park is limited so arrive in the park by 8am to get a spot. Otherwise park just outside the park entrance and walk in...or park in Silverdale and get the free shuttle. The shuttle buses within the party are great to take you through the park but lines form from mid morning onwards.
You could do the main hikes in two days, but we stayed for four, doing a hike each morning then relaxing in the afternoon; the hikes vary in difficulty but all are tiring after a while.
I found the hike up to Scout's Outlook the best but the extension up to Angel's Landing true to the category of strenuous. It is also dangerous but mostly because of the carelessness of others...it was each man for himself so to speak and it made me uneasy after a while so I turned back....only to see even more hikers making their way up so the journey back to Scouts Outlook felt more dangerous than going up. Just be careful if you attempt this hike and leave very early in the day to avoid the crowds.
Zion is a stunning and unique place. Totally recommended!!!