Monday is the best day of the week to get gas, according to a new study.

GasBuddy found that Mondays offer the lowest average gas price in 30 states.

That’s true if you’re filling up in North Dakota or South Dakota, but not true if you’re in Minnesota.

“If you’re in North Dakota on Monday, pick up some fuel, but if you’re driving toward Minnesota and can wait a day, pick it up on Tuesday there,” said Dan McTeague, a senior petroleum analyst for GasBuddy.

The only state where the best day to buy gas isn’t Monday or Tuesday is Hawaii, where prices bottom out on Wednesdays.

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According to AAA, the national average gas price per gallon is currently $2.78.

In North Dakota, the average price per gallon was $2.68 heading into the weekend. The average in South Dakota was $2.70 and in Minnesota it was $2.71.

McTeague said the national average gas price per gallon is expected to increase 20 cents by Memorial Day weekend.

“We’re going to probably hit close to $3 by then,” McTeague said.

The worst day of the week to fill up in both Minnesota and North Dakota is Friday, with the most expensive average price peaking toward the end of the week, GasBuddy found. Saturday is the worst day to refuel in South Dakota, as well as 15 other states.

GasBuddy analyzed gas price data from January to March 2019 from large metropolitan areas in every state, McTeague said.