Rattlesnakes will usually come out of hiding when it gets to 60 F or above and are usually most active during the temperatures of 80 F and 90 F and then if it gets hotter than 90 F the rattlesnakes will usually go back into hibernation in the shade until it cools off again.
Rattlesnakes as well as many other snakes are most active during the day and then go to sleep at night.
A rattlesnake is most active during the early morning hours during the spring and summer when the sun is warming the earth up.
If the weather gets too hot then sometimes a rattlesnake or any other snake will go into hiding in shade to stay cool and then might return later in the evening although most snakes including the rattlesnake will settle in for the night and then come back out the next morning.
I have seen a few rattlesnakes out before in the evening which is rare but most times I've seen them out in the daytime early morning hours.
Also when a rattlesnake bites someone they can only bite the person when they are in a coiled position so if the rattlesnake is not coiled up yet then they cannot usually bite you.
If you do get bitten by a rattlesnake though you need to get medical attention because the venom that a rattlesnakes injects into the person is deadly if not treated promptly.
Also if you or someone gets bitten by a rattlesnake you should immediately apply a tourniquet above the bite and ice it and get medical attention.