(GasBuddy) September 15 might not hold any significance for you, unless you were born on that day, have an anniversary or experienced some other life changing on that date.

But it is an important date when it comes to buying gas for you car or truck. It’s the last day the EPA requires the more expensive summer-blend gas. According to GasBuddy most refineries start producing winter-blend again in late August and will draw down the remaining summer supply. In addition, gasoline demand falls as temperatures begin to seasonally drop, leading gas prices to fall throughout the fall. Starting Sept. 16, gas stations can again start filling up with the non-summer, or “winter” gasoline.

What is Summer gasoline?

In the warmer months, gasoline has a greater chance of evaporating from your car’s fuel system. This can produce additional smog and increased emissions. Refiners reduce the chance of gas evaporation in your car during the summer by producing gasoline blends that have lower Reid vapor pressure (RVP), or lower volatility. These blends vary from state-to-state, region-to-region due to RVP state regulations. They also vary by octane level.

Cost for your wallet: According to NACS, this higher-grade fuel can add up to 15 cents per gallon to the cost of your fill-up. This excludes the increased cost due to summer fuel demand, which can vary between 5-15c/gal, depending on region. More stringent requirements (like California) can mean an even higher cost.

What is Winter gasoline?

In winter, gasoline blends have a higher Reid vapor pressure, meaning they evaporate more easily and allow gasoline to ignite more easily to start your car in cold temperatures. This blend is cheaper to produce, which results in lower gas prices at the pumps from late September through late April.

Cost for your wallet: Prices typically fall 10-30c/gal starting in mid/ late September through late November as gas stations switch to winter gasoline and demand for gasoline falls seasonally as we start to stay closer to home. Many retailers continue to sell summer gas until their inventories run out before then selling winter gasoline. Hurricane season can also affect prices before the switch, ending in a squeeze just before the switch since refiners don’t want excess expensive summer gasoline sitting around and especially if refineries are in the path of a major storm.