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.

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That's true if you're filling up in North Dakota. But perhaps not true if you're across the river 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.

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

In North Dakota the average price per gallon was $2.67 heading into the weekend.

The average price per gallon in Grand Forks County was $2.66 Monday.

McTeague said the national average gas price per gallon is expected to increase 20 cents per gallon 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 states is Friday. Friday has the most expensive average price compared with all other days of the week, GasBuddy found.

Saturday was the worst day to buy gas in 16 states.

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