The correct answer is - equinox.
The equinox is a natural occurrence when the day and the night are the same length. The equinox appears twice a year, in the spring, thus the spring equinox, and the autumn, thus the autumn equinox.
The spring equinox marks the beginning of the spring, and it occurs on the 21 March, while the autumn equinox marks the beginning of the autumn, and it occurs on the 22 or 23 September.
Both equinoxes occur because of the inclination of the Earth towards the Sun. Note that the spring equinox in the Northern Hemisphere is actually the autumn equinox on the Southern Hemisphere, and vice versa.
