Fall begins at the autumnal equinox. What is the autumnal equinox and why is this the start of fall?
Models of the earth always show it tipped. Look at a globe and the rod it spins on is tipped. There is no rod for the real earth to spin on. The imaginary rod the earth seems to turn on, called the earth's axis, is tipped. As the earth moves in its orbit around the sun, sometimes the northern half of the earth tips towards the sun to give us more hours of sunlight and stronger sunlight. That's our summer. Sometimes the northern part of the earth tilts away from the sun to give us fewer hours of sunlight and weaker sunlight. That is our winter.
At two times in the yearly trip around the sun, the earth is lined up so the axis is tilted exactly sideways to the sun. When this happens, every part of the earth gets 12 hours of sunlight and 12 hours of night. These two days are called the equinoxes. One is in the spring and is the official start of spring, the vernal equinox. The other is in the fall and is the official start of fall, the autumnal equinox.
So, around September 22 each year, the earth's axis will not point towards the sun or away from the sun. In North America and all other places north of the equator this will be the autumnal equinox, the start of fall. It is the date when the earth is one half of the way between where it was at the start of summer (the summer solstice) and where it will be at the start of winter (the winter solstice).
To learn more about the cause of seasons, visit our page on What Causes Winter. It tells how most drawings of the earth's orbit are wrong or at least confusing. The seasons are not caused by the earth being closer to or farther from the sun.
Windows to the Universe has an explanation without animations.