courtesy: Norwegian Air has released a study that says the best and cheapest time to buy airline tickets online is 54 days before travel for deals on domestic flights.

The study also says Sunday is the most expensive day to fly and Tuesdays and Wednesdays are the cheapest–an average of $73 cheaper.

The worst time to buy a domestic airline ticket is during what the company calls the “Hail Mary” window, 0-2 days before flying when a ticket price can go up over $150 more than during the prime booking window of that 54 day period.

The study also found that the lowest fare for a given flight changes an average of 71 times between the time it’s announced and the day the plane takes off. The price of a flight changes on average every 4.5 days, and each change averages $33 up or down.

“The most important rule is fairly obvious: don’t wait until the last minute, as that rarely works out. But beyond that, you also want to be careful not to buy too early. I always suggest that travelers check fares early and often and get familiar with the market. Then, when you see a good deal pop up, grab it, because it likely won’t last very long,” said CheapAir’s CEO Jeff Klee in a release on the findings.

Best practices not in the study also suggest that you should not only visit booking engines like CheapAir but also check the airline’s own site where pop-up sales are increasingly posting for low-cost carriers like Norwegian.

Be aware, too, that rock-bottom cheap flights may mean departures and arrivals into smaller airports that are further away from metropolitan areas.  Add gas or public transportation to the price of those tickets and the cost goes up.