The only way you could reduce demand is if people drive less. So not buying gas on a single date would not do much, if everyone still drives the same amount they would on that day. Cuz they are gonna still going to make up for the gas they used up on May 19th, on a day other then the 19th, unless everyone did not touch their cars on that day. On the other hand, it could work by drivers making a statement to oil companies, forcing them to increase in the refining of crude oil into gasoline and also using other tactics to decrease the price of gas. But then again there are other factors other then supply and demand that go into the price of gas, (a.k.a. threat of terrorism) which could act as a premium on the price of crude oil, some analysts say it raised the price of a barrel of crude oil by as much as 5-10 dollars per barrel.