Of course, by artificially and temporarily subsidizing a lower price for gasoline, you'll be encouraging people to buy more of it, which increases demand, which will lead to even higher gas prices when the "holiday" ends. It's kind of like offering a junkie free crack on Mondays for a limited time only.
In an economist's ideal world, gradually rising gas prices will gradually make people reduce their consumption until prices cease to rise anymore. Some commodities experts estimate that this may happen when the price of gas tops out at about $10 a gallon (see this New York Sun article).
But if the government meddles in the meantime, Americans won't be as inclined to conserve, which means that gas prices will continue to rise, and at a faster rate than they would without the tax "holiday." And when the "holiday" invariably ends, motorists will be whammied with a sudden 20 cent jump in prices. A summer-long gas tax holiday seems like a great way to guarantee $5 a gallon gasoline just in time for the fall hurricane season.
So gas tax holidays invariably lead to nasty gas price hangovers. Even I, who am no fan of happy motoring, find this policy a bit sadistic - especially since the prime beneficiaries of high prices and unrelenting oil consumption are companies like ExxonMobil, pulling in eleven-digit profits.
At the same time, the federal government's highway fund will lose billions of dollars for every month of gas tax vacationing - which also means that the federal capacity to fund new highway and road projects will wither away just as rapidly. The national highway trust fund is already running short of money to pay for big new freeway projects, so this could make it even more unlikely that the federal government will be able to afford to pay for new freeways and highways in the future. Fine by me!
Last night, a bunch of Maine truckers drove their rigs 800 miles to protest in D.C. At about 10 miles to the gallon, each truck spent over $300 for the one-way trip (not including engine wear and the opportunity costs of lost revenue). It probably wouldn't make them feel any better to point out that they could have spent less than half the money for a train or bus ride to Washington...