I believe it is sept 23.

As a plant live cycle season it varies a lot with the latitude - starts sooner the further south you are. As an astronomical event, it starts at the same time as spring in the northern hemisphere, in march at the equinox (varies slightly - around march 20 or 21). It marks the beginning of the decrease in daylight in the southern hemisphere and the increase in daylight in the northern hemisphere.

for bergler83

You are wrong my friend. That is when the autumn starts in the northern hemisphere. It is in march in the southern hemisphere. Read answer to main question above.