Driving style (lead foot? driving mostly local?), road condition (stuck in traffic a lot?), as well as weather (A/C on or not) all can contribute to MPG.
Are you monitoring MPG using the car's driving history or by how much you fill each time and miles to next fill? Don't know where you're at, but there's an issue with a certain "off brand" chain of stations fiddling the pumps here in No-CAL, and possibly elsewhere. They pump less gas than the dial says so that throws off MPG calculations, not to mention the wallet too.
Checked tire PSI all way around? Don't just rely on the TPMS system...
Extra weight (more passengers, luggage, etc)?
Any codes set for O2 sensors (eg, rich fuel), etc?
Since you are asking in September, perhaps one of two things. One, schools are back in session, and that tends to add more "pressure" to the traffic patterns. Two, traditionally, gasoline formulations start to change from summer blends to winter blends. Might be one or both these reasons. I can certainly vouch for the "school" reason since school busses roam my neighborhood every weekday morning and afternoon, where I need to wait for the kids to get on or one or more buses, and traffic in general is heavier when schools are open. My other suggested reason, gasoline formulation changes, might be a little early. Someone who is in the gas business can feel free to correct me if I am wrong about exactly when this happens.
I mention these two items because my mileage has certainly gone down about 5 mpg since late August.
Check the brake Calipers, I had this on my accord (although different Gen) See after long drives if any of the wheels feels hotter than the other. If so, thats the caliper thats sticking.
Also a nail in the tire causing underinflation would cause this too.
I have noticed a drop as well, but I switched daycares to a place closer to home. My miles driven and gallons burned overall have decreased, but so has my efficiency.
If you follow my excel chart, the gray gallons/day line follows the RIGHT Y axis, while mpg and miles/day follow the LEFT Y axis. The closer miles and gallons/day are, the better my efficiency. The last 3 fillups happened after switching to a daycare that is directly on the way to work, instead of far out of my way. That and temperature are the only changes so far, they should be rolling out winter fuel in the next month.