It’s 2015 and the decade-old conundrum still exists: how do we find the cheapest flights? Judging from Expedia’s most recent study on 2015’s flight trends, it’s clear that several factors culminate to influence travel fare.

Previous studies have revealed Tuesday to be the optimum time to buy airline tickets, offering the cheapest prices on average. Expedia reports that this remains true, though the margins have slimmed. Additionally, this statistic is only relevant with regard to tickets booked at least three weeks in advance. Though the study does go on to acknowledge that ticket price is an ever-changing variable that can be only loosely predicted by daily averages.

With flash sales and bargain hunters, you can never be sure when you may stumble upon the most inexpensive airfare. Most advertised airline sales are launched on Tuesday, though there are unadvertised sales that can often lead to bigger discounts and can be found on any day of the week. Add into the mix travel agents and travel consultants who have relationships with airlines and can scour the marketplace for even better deals, it is hard to say when and where you will find your next airline promotion.

Expedia does, however, emphasize the importance of which day you fly. Long-haul flights departing on Thursday and returning on a Monday are significantly less expensive (nearly 20%), than its most expensive counterpart: Friday to Saturday. Alternatively, in considering short-haul flights, flyers will save 25% with a Saturday departure and Monday return, versus the extremely pricey Sunday to Monday excursion.

And finally, advance ticket purchase offers a clear-cut advantage when trying to keep prices at bay. For domestic flights, 57 days in advance is the ideal time to purchase, whereas that number goes up to 171 days with international bookings.

So what advice should you take away?

Book early

While you need much more preparation for international flights, stick to roughly two months in advance when purchasing your domestic flight.

Flexible dates over the weekend will drastically decrease total fare

When booking your flight, toy with departure and return dates. If you’re flexible, a Thursday departure could save you hundreds.

  Check fares on a Tuesday

Widely accepted as the best day to book a flight for years, though don’t sweat it if you can’t, this statistic is often negligible.