The travel website Expedia always has our backs when it comes to saving on travel. Recently, Expedia taught us that the best deals on hotel rooms come when you book on Sunday . Now, it's giving us the inside scoop on what days are best for booking flights.This week, Expedia released a report analyzing air-travel data from the past year to help consumers understand ticket-price patterns. Using information from airlines, the site was able to figure out which days you can find the cheapest airfare. According to the report, average ticket prices are highest on Fridays and lowest on Saturday and Sunday. This trend is due in major part to the fact that business travelers tend to make big purchases at the end of the week. Expedia even reported that if you're booking a flight into the United States, Sunday ticket prices are almost 50% lower than Friday's. So next time your planning to fly, it might be worth it to take some time out of your weekend to purchase the plane tickets.In addition to looking at what day of the week has the lowest airfare, Expedia also looked into how far out you should book your trip. It found that buying your plane ticket 21 days or more in advance gets you the lowest average ticket price. This is especially true when booking travel to Europe or within the United States. With these tips, we can spend less money on flights and more on adventuring once we get to our destinations.