Question: When is the best day to fill my tank? Dr. David Rizzo: Wednesday is the best day, here is why. Anecdotal evidence that comes in from readers and motorists on road via cellphone and websites shows that gas station tend to raise their prices on Mondays and Thursdays, so you want to beat them with a punch, Wednesday is a good day. You certainly never want to wait till the weekend and if it's a three day weekend, definitely fill up earlier because the gas station owners know that a lot of people will be hitting the road, they raise the prices. So, Wednesday is a nice, happy medium to get the best price.