What's happening is that the earth is passing through the orbit of past comets, which happens multiple times each year. Since we base our calendar off of the earth's position in its orbit, we pass through the same spot each year at the same time, plus or minus 12 hours depending on the leap year cycle. The meteorites, just mass that separated from the comet, are just circling in orbit like any other body of mass, but the earth passes through the orbit, so they get sucked in by earth's gravity, and there you have your meteor shower. The meteor shower appears to be only at night, because the sun light and the refraction in the atmosphere prevents us from seeing it during the day, but they typically occur for several days continuously.