When do rattlesnakes usually start appearing?

I’ve got a toddler who loves running around our backyard which opens up to a lot of land. It’s been nice not having to keep as close vigil for snakes, is there a rule of thumb for when they start appearing? I don’t know if it’s a matter of when the night low temperature rises above a certain level, or the highs get past certain point, curious if any snake experts know…