The Equinoxes are usually thought of as times of balance. A time when the powers of light and darkness are equal, and thus a moment of harmony within the cycle of the turning year.
At the point of the Equinoxes the length of day and night are equal.This is easily depicted as an old fashioned balance scale, the symbol of Libra (Sept 23-Oct 22) with the two pans of the balance level.
But a more accurate analogy might be the eye of a storm. When the 'eye' of a storm passes directly overhead, there is an eerie time of peace and calm before the storm moves on and hits with its full force again.
The wheel of the year never ceases its turning, it is constantly and perpetually in movement. If you consult an astrological ephemeris you will see that there are precise times given for the Equinoxes (and Solstices too). So rather than being thought of as times of balance, it might be more accurate to think of the Equinoxes as times of dynamic change.
They are certainly times which are known as 'cracks between the worlds', special times of magical energy, but they are times when the emphasis or direction of those energies undergoes fundamental change.
At the Autumn Equinox suddenly the emphasis of the energy changes from more-light-than-dark to more-dark-than-light.
Coming up to the Equinox you can almost see this change happening every day. Each day the sun rises later and sets earlier. Each day the darkness lingers longer in the morning and closes in more quickly in the evening.
If you are tuned in to these magical currents, as most magical practitioners are, you can feel the rising of the dark energies.
By the way NEVER get into the mind set of light = good, dark = evil. These energies are Yin and Yang, you can't have one without the other. It is always what you do with those energies that is important. You might as well say electricity = good, magnetism = evil. They are just energies.
BUT to a magical practitioner, often the energies of the dark half of the year are more useful and more powerful.