However, get medical help right away if you notice any symptoms of a serious allergic reaction, including: rash, itching/swelling (especially of the face/tongue/throat), severe dizziness, trouble breathing, kidney problems (such as change in the amount of urine). This is not a complete list of possible side effects.
They are more active when it is cooler, but they can and do come out of hiding at any time of the day. Here in the desert, Arizona Rattlesnakes can easily overheat and die if they cannot find a cool place to keep themselves cool.
I live on the north side of North Table mountain. All kinds of rattlesnake reports come from there. And about 10 years ago, the Lakewood police chief, while hiking on Green Mountain, was bit by a rattlesnake. According to my veterinarian, between March and October, rattlesnakes can be found anywhere in Colorado at elevations lower than 8500 feet.