Snakes are seen just about everywhere from spring through October, and then they are presumed to hibernate. But that's not always true. Snakes are more reactive to consistently cold weather, rather than a change of seasons. Rattlesnakes are even known to move around during extended warm periods during winter months, especially if they hole up in a stump or other location that can be warmed easily by sunshine. 
Snakes do not actually hibernate, rather they become less active during cold weather. It is called "brumation." Brumation is an extreme slowing down of their metabolism. Snakes are awake, but just very lethargic so you don't see them moving around. In the fall, snakes move back to the previous year’s den. If a sudden cold snap catches them before they get there, they may die if not fortunate enough to find a suitable secondary den. They usually do not stay long at the den entrance, but hurry in for the long winter sleep. A number of species may share the same den. For example, black rat snakes, timber rattlesnakes and copperheads commonly den together. Sometimes there will be as many as 100 snakes in one cave. A group site is called a hibernaculum.
Cold-blooded animals like snakes, fish, frogs, and turtles need to spend the winter inactive, or dormant, because they have no way to keep warm. Snakes will crawl into any area free from frost such as caves, hollow logs, holes under trees and stumps, under wood piles, in other animal's burrows, and occasionally in a person's basement.
Snakes will increase their intake of food before brumation occurs, if they can. Not all snakes will survive brumation. A skinny snake will not survive. If the snake feeds heavily before they hibernate, and have digested their meal before the cooling starts, they will be OK. If food is in their stomach or intestines when they cool, it will rot and kill them. Vipers can also brumate during normal conditions, due to a loss of food, but normally when they become dormant it corresponds with extreme temperature changes.
On warmer days in late fall, early spring, or even during winters, brumating snakes sometimes come out of their dens to bask in the sunshine. Nice sunny days that follow a long cold snap are often when people are surprised by rattlers. Just like humans, snakes head out to enjoy the sun and unsuspecting hikers can startle them and cause them to strike. Generally, rattlesnakes emerge from hibernation in March or April, or when the average daytime temperatures reach and remain about 60 degrees Fahrenheit and higher.
During a typical year, an estimated 45,000 people are bitten by snakes in the United States. That number is reason enough to always wear snake boots or snake gaiters when in the desert or the woods. If you do, you won't have to worry so much about the temperature or the season — use common sense to keep yourself safe. Hunters should be especially vigilant and wear snake protection!
