For those who watch the calendar, today marks the official end of summer and the beginning of autumn with the equinox today. It began today at 07:50 UTC (go here to see the time it began in your area). There are two equinoxes in the year: March and September. 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 while in the South it is the beginning of spring.
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.