The Earth orbits the sun once every 365 days and rotates about its axis once every 24 hours. Day and night are due to the Earth rotating on its axis, not its orbiting around the sun. The term 'one day' is determined by the time the Earth takes to rotate once on its axis and includes both day time and night time.
Day and night occurs because the Earth revolves around the Sun, and when the Earth revolves, different parts of the Earth get sunlight at different times. When it is night in one place, it is daylight in the other. This is because we only have one Sun and wherever the Sun is, that is where it will be bright.
