It occurs twice a year, once in the Spring and once in the Autumn - but the Equinox is not when day and night are exactly equal.
Let's start with the basics...
What is the Equinox?
The Equinox in the Northern Hemisphere occurs twice a year around 20 March (the spring equinox) and around 22 September (the autumn equinox).
They occur between the summer and winter solstices marking the point the Sun crosses the equator's path and becomes positioned exactly above the equator between the Northern and Southern Hemisphere. During the equinox, day and night will be around the same length which is evident in the word's origin derived from the Latin equi (meaning 'equal') and nox (meaning 'night').
The spring equinox marks the beginning of astronomical spring and from this day forward the day is longer than the night. Similarly, the autumn equinox marks the start of astronomical autumn as the night becomes longer than the day.
What is the Equilux?
The equilux is when day and night are equal and occurs a few days before the spring equinox, and a few days after the autumn equinox.
On the equinox, the length of day and night are only nearly equal. This is because the Sun appears as a disk in the sky, and the top half rises above the horizon before the centre. As well as this sunlight is refracted by the Earth's atmosphere. The Sun, therefore, appears to rise before its centre at the horizon, giving more daylight than you might expect (12 hours 10 minutes on the equinox).