By May 1, petroleum refiners are required to have switched to summer-grade gasoline. Summer gasoline is required to have lower volatility, which means that evaporative emissions are reduced. If the switch was not made, our cities would have more ozone and your engine might suffer from vapor-lock as well.
The summer grade gasoline is required to be at your local gas pump by June 1. (California is required to start earlier.) You will also notice a bump in the price - summer grade gasoline costs more to produce.
Date of switch to summer-grade gasoline approaches tells that winter-grade has more butane. You will notice in the map that high population density regions such as Atlanta, Tampa, Miami, St. Louis, and the northeast corridor are required to even cleaner, reformulated gasoline RFG. The EPA has required even less volatility in the gasoline for these regions to reduce smog-forming particulates and pollutants. Naturally, expect to pay more for the privilege.
Speaking of gasoline, has it seemed harder to find a gas station around you? Gas stations are vanishing from Washington's inner suburbs explains it isn't your imagination. Rising property values and increasing wholesale costs for gasoline are pinching gasoline retailers. Because the profit margin on gasoline is so low, many have almost become a small supermarket and car wash to be economically viable. Many gasoline stations don't have that much real estate, or if they are in places like D.C., the property may be worth more than the business. Hence, you may have to start driving farther to fill up your tank.