Even if you’ve only driven an automobile a couple of years you’ve probably noticed a pattern in gasoline prices. Price goes up in the summer and down in the winter with fluctuations in between.Its partly due to supply and demand.The other part is probably OPEC.
I’m wondering if there has ever been a study as to just when these increases /decreases occur.
Is it a state by state or region by region thing?
AFAIK, the official explaination for why gas prices rise in the summer is that’s when most people go on vacation, and since many of them drive, the increased demand helps drive up fuel prices. The same thing happens around the holidays.
True but it also happens during planting and harvest seasons when a great deal of fuel is needed.
We have a local happening here in Mt Pleasant Iowa called the “Midwest Old Threshers Reunion.” It happens during the Labor day holiday. I’ve noticed gas prices raise before the reunion but when its time for all those folks to go home prices drop.
Just a local thing or nation wide?
Here are some data points, if you care to plot them out: 
The average price of gasoline, by week, since 1991: http://www.eia.doe.gov/oil_gas/petroleum/data_publications/wrgp/mogas_history.html
The average price of oil, by week, since 1946: http://research.stlouisfed.org/fred2/data/oilPRICE.txt
Generally, the price of gasoline tends to peak in August-September.
If you take the Excel data from the above spreadsheet and plot it out, you’ll see that the price does tend to peak in August/September and bottom-out in January.