Snakes are not particularly nocturnal or diurnal; they come out when they can. Because these reptiles are cold-blooded, their body temperature rises or falls depending on their environment. This means that snakes must come out and hunt when they can, because if it is too hot or cold, they cannot survive.
When it is extremely hot or cold outside, snakes tend to be inactive and find a place to cool off or warm up until the temperature becomes milder. These reptiles do not have a set time of the day or night when they are active though; it is just a matter of what the temperature of their environment is like.