If you track airfares regularly, you’re probably convinced prices are set by a group of chimpanzees throwing darts at a newspaper somewhere outside Toledo. That seems like the only way to explain why a flight from Chicago to Dallas costs $300 on Friday, but $150 on Saturday. Airfares can’t be explained by simple economics, but is there a consistent day of the week where airfares are cheaper? And are there days that are cheapest to fly for specific holidays?

The cheapest days of the week to travel

Most data show that Tuesday through Thursday departures are the cheapest for domestic travel, with Tuesday the cheapest of all. This is according to the seminal research done on the topic, Expedia’s 2015 white paper “Preparing for Takeoff: Air Travel Trends in 2015.”

Breaking it down further, for short-haul flights, Monday and Wednesday were the next-cheapest days to fly after Tuesday. Long-haul flights, however, are slightly more complicated: Thursday is the cheapest day to fly out on a long-haul flight, while Monday is the cheapest to fly home. Saturday is the most expensive day to fly both in and out on long-haul flights, unless you fly Saturday to Saturday, which is the second-cheapest itinerary, after Thursday to Monday. If you’re confused, just re-read all that slowly.

Travel search engine Kayak also notes that for domestic flights, Sunday is universally the most expensive day to travel, but for international routes, weekdays are more expensive. So flying to Europe on the weekend and to California during the week will save you the most money.

The cheapest days for holiday travel

When it comes to holidays, the general rule is the closer to the holiday you fly, the cheaper the ticket.

For Thanksgiving, Thanksgiving day is the cheapest time to fly out, and Black Friday is the cheapest day to fly home, according to Cheapair. The Tuesday after Thanksgiving is the next-cheapest for return flights, and probably beats flying home for dinner and leaving the next day.

For Christmas, the cheapest days to fly out are the 23rd to 25th of December, with December 31st the cheapest day to return.

For summer travel, Kayak found the cheapest days to fly for July 4th are July 2nd for departures and July 5th for returns. Labor Day, since it’s always a Monday, follows a more-traditional pattern, with Saturday departures and Tuesday returns the cheapest.

Of course, none of this is an exact science, and you may stumble upon a freak week where Tuesday departures are in high demand, with dirt-cheap weekend flights for no good reason. But by following the trends, you can usually save money.