September means it is time for tweed. The lightweight variety of tweed, that is.
Now, there are cloths that look like tweed and are lighter than the 14 ounces/420 grams cloth that is the lower boundary of the real thing, but all incorporate some form of compromise. For example, the excellent 12 ounce/360 gram Porter & Harding Glorious Twelfth patterns are worsteds posing as tweed. That does not make them somehow inferior, but they are not tweed.
Another approach to lighter tweed is to use a 2x1 weave instead of 2x2, with or without a worsted yarn in combination with the woolens. That makes for lighter weight but also greater delicacy. Still, it has its place, so long as there are no expectations that a jacket from the stuff will be passed down to future generations.
In the photograph, a 10.5 ounce/310 gram herringbone combines with a (not so) hidden buttondown chambray shirt and the first wool tartan necktie of the season.