Carol Leigh's CalPhoto.com Wildflower and Fall Color Hotsheet. (Site down due to disregard for private property 3/30/10)
Finding a source that is up to date for spring wildflowers and fall color in your area is a great tool and increase your chances of getting there at peak season. If you are not in California most states / provinces usually have something similar or even multiple sites that can help you find what you are looking for. Just keep in mind the greater the elevation change you have the longer your season will last. Here in Southern California fall ends and spring can start in only weeks. In the rest of California you can find either one or other, something blooming or leaves changing color during anytime of the year.