An equinox is a period in the orbit of a planet in which the planet's orbit and position cause the Sun to pass directly over the equator. In addition to being an astronomy event of note, it also marks a shift in the seasons. On Earth, equinoxes appear around 22 September and 20 March every year, although the days can shift slightly, especially since the date is calculated using Coordinated Universal Time (UTC), which means that equinoxes in the Far East can fall a day or more later than equinoxes in Europe and the Americas, because of time differences.
Every planet has a center line or equator, along with a celestial equator, an imaginary plane created by moving outwards from the equator. When the equinox occurs, the Sun is directly over the celestial equator, which means that someone standing at the equator would be able to see the Sun immediately overhead. It also means that half of the planet is fully illuminated, and that day and night are of roughly equal length.
Given the name, which involves the root for "same," one might be forgiven for thinking that day and night are of equal length at this point in the year; however, this is not actually the case. A number of factors can influence the length of daylight experienced in any given area, and the precisely equal days and nights actually fall slightly ahead of the spring equinox, and just after the fall equinox. Two people standing at equal distance from the equator would experience days and nights of the same length, however.
For people at the North Pole, the spring equinox signals the start of six months of near continuous daylight, while people at the South Pole have six months of night to look forward to, as their fall and winter are just beginning. On this one day in the calendar, however, both see equal amounts of sunlight, even if it signals the start of different seasons.
Another interesting event occurs at this time: the Sun rises due east, and sets due west. Many cultures have associated specific meanings with these times of year. These dates traditionally mark the changes of the seasons, just like the winter and summer solstice, which mark the shortest and longest days of the year respectively. Festivals have often fallen on or around these dates historically, and people may use the event as a handy reminder to perform routine maintenance on homes or cars.