There are two equinoxes in the year, Autumn (September) and Spring (March). When these equinoxes occur the sun is directly on the equator, and the length of day and night is almost equal. In the Northern hemisphere, the September Equinox heralds autumn but the opposite below the equator where it heralds the beginning of spring. The Autumn Equinox begins today at 19:21 UTC (go here to see the time it begins in your area).
For those of us in the North, it means a transition from summer to winter. During this period days start getting shorter and nights longer. Depending on where you live, you will likely have moderate warm days followed by long and cooler nights. Harvests of many crops often take place during the fall and in the old days you would make preparations to store food for the winter. Harvest festivals are very popular and in particular Halloween. Pumpkins begin appearing along with all kinds of Halloween decor culminating, of course, in All Hallows Eve (Halloween) on October 31.