Have you noticed it is getting dark earlier each day? The sun is also coming up a little earlier. Between the two, we are loosing a few minutes of daylight each day. By the end of the month, we will lose more than one hour.
Also, we can expect a big change in temperatures. After starting the month with highs near and even above 90, by the end of the month we could see a day stuck in the 60s if we get the right combination of clouds with some rain. Many years we dip into the 40s a time or two in September as well.