[mrtexas]

I'll act like this is a serious question rather than bait to attack oil companies.

Generally in the spring. It depends where you live. Spring is earlier in the Southwest than it is in New York. There are cutoff dates and a transition. The only difference is the amount of n-butane blended. Summer grade has little to none these days and winter grade 5+%. Winter grade has more so that your car works in 0F weather, like 15psi in the winter and 7psi in the summer. The other spec is the T V/L=20. This is the temperature where the vapor/liquid ratio of the sample is 20. Winter grade is somewhere around 105F and summer grade is like 140F. Intermediate grades are well somewhere in the middle. The 50% distilled point stays the same with a maximum of around 250F.

One thing the EPA couldn't legislate is the laws of nature. It takes a more volatile gasoline with more butane to vaporize in the intake manifold in cold weather.