For the most part, these birds are active during the night as you could probably guess. We usually hear them doing their "peent" call right before dawn while we are setting up our mist nets. And they also seem to like to sleep in the road while I'm driving up to our banding sites. (Don't worry they always fly away when they hear the car) Once in awhile we also see and hear them well into the daytime. Then they transform into Common Dayhawks, of course.
If you are enjoying a summer evening, remember to look for the nighthawk's long dark wings with white central spots. Or listen for the nasal "peent peent" call coming from the sky. Or I suppose just look out for any bird shaped tree branches during the day.