Gasbuddy.com's Patrick Dehaan says it can vary from state to state.
"When we talk about Texas the cheapest day is Monday and the most expensive day is Saturday."
But don't panic, it's not much of a price difference.
"There are sometimes where Monday could be several cents cheaper and there are somedays where it might not be, but overall, you end up saving about a penny and a half and that's a pretty good average."
Dehaan says the oil and gas markets are closed over the weekend and some station owners also expect business to drop from the weekend, so that may be why they lower prices on Mondays.
"Energy markets are closed over the weekend and because the price of wholesale gasoline doesn't fluctuate station owners may be more likely to take a risk and drop prices, knowing that they may have some room to go down."
Gasbuddy says Monday is the best day of the week in 23 states, including Texas.