Rattlesnakes are most commonly encountered from spring through October, and then they are presumed to hibernate. But that's not exactly true, depending on where you live. 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 become sluggish anywhere below 60 degrees° F, so as long as warmer temps prevail, as it seems to be doing this Fall, please be diligent and wear snake gaiters (lower leg protection) when outside raking leaves, stacking firewood , hunting, or hiking. A steady temperature drop is a signal for the snake to enter brumation (similar to hibernation), but that’s not nessarily happening yet.
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 late Autumn, 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, barn, or outbuilding. 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 October and November, early spring, or even during winters, brumating rattlesnakes 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 their pseudo-hibernation in March or April, or when the average daytime temperatures reach and remain about 60 degrees Fahrenheit (16 degrees Celsius) and higher.
The Centers for Disease Control and Prevention (CDC) estimates that 7,000 to 8,000 people get bitten by venomous snakes each year in the USA (mostly by the Western Diamondback Rattlesnake, Western Pygmy Rattlesnake, Timber Rattlesnake, Cottonmouth, Copperhead, and Coral snake), resulting in 8 to 15 deaths. That number is reason enough to always wear 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.