Zion has a rich history, certainly visited and inhabited by early Native Americans. The first Europeans to visit the area likely visited in 1776, and by the mid 1800's, Mormon farmers and families began settling in the region. In 1909, the area was afforded official government protection as Mukuntuweap National Monument, later re-named to Zion in 1917, and made an official national park in 1919.
Today Zion National Park is a world renown tourist destination. Currently, visitation is over 2 million visitors a year, an astonishing number!
Zion can be visited year-round for sightseeing.
For canyoneering in Zion, early summer through fall are the preferred seasons (June through October). Late fall, winter and spring can find canyons full of snow and ice, and dangerously impassable. Summers are a paradox in Zion. It is often well over 100 in the sun, but water temperature in the canyons can still cause hypothermia in a matter of minutes.
For hiking and climbing, spring and fall can be delightful, while summers are brutally hot. Be careful in late fall or early spring, however, as some trails/climbing descents can have snow and ice.
Canyons don't receive much sun, and can hold substantial water. If a canyon description recommends a wetsuit, that recommendation applies even on a 100+ degree day! A full wetsuit is likely the most critical piece of gear. For longer canyons, such as Heaps, Kolob, and Imlay, bring a full 5mm+ wetsuit. Two piece suits work really well, and offer double layer protection over the body core. Some use drysuits, though I am not a fan. They are prone to small punctures and tears in the canyon, and require wearing insulating clothing underneath that can absorb water when a leak occurs. The last thing you want half way through a long day is to stop, take off your drysuit, and find/patch a hole.
Potholes are also an issue with some Zion canyons. If a description requires pothole escape skills, make sure you have the skills and tools to escape them. Variable water conditions can turn a casual canyon at full water levels into a physical and challenging pothole escape obstacle course at lower water levels.
For better or worse, Zion has a permit system for canyoneering, some hiking trips, and all overnight climbs. Details are available on the Zion National Park website. They occasionally change, so check there before planning a trip.
As of 2011, popular canyons can be difficult to get walk-up permits for on weekends, however, during the week there is generally much more availability. If you are planning an extended trip to Zion, doing popular canyons during the week, and less popular ones on the weekends increases your odds of getting permits. If getting walk-up permits, it's best to have several canyons in mind and take whichever permit you can get. Reservations can be made online in advance, but the Subway and Mystery canyon require the use of a lottery system, and other permits can sell out online quickly.
Zion has a diverse set of canyons, some of these are well suited to beginners, but some are not. Do not venture into the harder canyons until you have amassed the skill and fitness level to descend them. There have been a number of fatalities and many very close calls by canyoneers lacking the experience and/or fitness levels for the challenges Zion offers.
In the same vein, partners matter! Harder canyons in Zion are not suitable to beginners or intermediates, even with competent leadership. Heaps, Imlay, Kolob, etc... require everyone in the group be skilled and competent.
Have fun, be safe! Zion has a great SAR team, but you don't want to get in a situation where you need them.