One day means the time the Earth takes to rotate once on its axis.This includes both day time and night time and it is approximately 24 hours.We get day and night because the Earth rotates on an its axis and during this rotation of 24 hours different parts of the planet are facing towards the Sun or away from it. Hence different parts of the world experiences day and night at the same time.Days are longer during summer and shorter during winter.
The day and night occurs due to the Earth's rotation. The day starts when the North Pole is facing toward the sun, and the night starts when it is facing away. This happens because of the axis of our planet's rotation. Some people call it "the wobble" because it actually takes nearly a year for us to travel one million miles but we only notice it in a day or two or three at most. It gives us around twenty four hours of light and twelve hours of dark as we spin around on our world, slowing down some over time as gravity pulls on us more than others while still carrying us with each orbit around our sun.