The equality of daytime hours and nighttime hours happens only twice in a year. Once during the autumn and once during this time the spring. Spring Equinox falls on 21st of March every year. It is the point of time when the sun is directly above the equator. It means that the day and night are equal. That means that number of daytime hours is equal to night time hours, twelve hours each.
Tilt of the earth’ axis
The earth is tilted on its axis by 23.5 degrees. That creates two different seasons rather opposite seasons in the northern and southern hemispheres. The tilt creates different seasons along the revolution of the earth with the sun at one foci. Taking the northern hemisphere into consideration, this part is more exposed to sunlight than it is on the equator. That means that the northern hemisphere has more daytime than nighttime.
This creates the hot and warm seasons in the Northern hemisphere. As the earth progresses in its elliptical orbit, the angle of the earth facing the sun changes and the sunlight gets directly incident on the equator. This is the equinox, the daytime hours are exactly equal to nighttime hours. Then henceforth, the southern hemisphere will start getting more sunlight creating the summer in the south and ushering winter in the north. This continues for 6 months and then again there is an equinox and then the summer in the northern hemisphere in initiated and winter comes to an end.
Fixing the date of Spring Equinox
Astronomy has been one of the preoccupations of the ancient civilizations. The astronomer in the past fixed one day while the astronomers in the modern have different dates for equinoxes and solstices. This is solely because of the calendars they followed.
According to the Julian Calendar established in 45 BC, the equinox was fixed at 25th March, but the Julian calendar was of a longer time frame, it had more time in it that it took the earth complete one complete revolution. That is why in 300 AD, the actual equinox happened on 11th March. Finally, it was Pope Gregory who finally created a calendar in 325 AD, based on the actual time the equinox occurred and actual time of earth revolution. The date was fixed at 21st of March. There is minor variation of +-27 hours in every 400 years. That is quite acceptable.