Planning your 2013 holidays? Often the timing of when you book matters more than which agency you use.

Historically, January is the month when tour operators, both online and offline, run the greatest number of deeply discounted holiday packages, where flights are sold along with hotels. Many of these packages are for travel between now and September, though some are only for trips within the next few months.

When it comes to purchasing flights separately, though, booking far in advance isn’t always the best idea.

Many studies – mostly focusing on tickets sold by US online travel agencies – have been done on the best time to purchase flights, and sadly, the results aren’t as clear and definitive as one might hope. But the overall direction of the research does offer a few clues to help travellers take advantage of the periodic rises and falls in airfares, both for travel in the US and beyond., the metasearch website, studied more than a billion US domestic airfares available for purchase throughout 2011. The site found that the average price when tickets were booked six months before departure ($406) tended to be 9% higher than the average airfare available during that six-month period ($370), and 19% more than the average price available a mere 21 days before departure ($342).

But don’t wait too long. As any savvy traveller knows, prices tend to skyrocket for truly last-minute purchases. The study revealed that airfares sold one week before departure tended to be 30% more expensive ($445) than they were two weeks prior.

For international trips departing from the US, timing didn’t matter as much. Tickets were the cheapest 34 days before departure, but the fares averaged $977, which is merely 4% lower than the average of fare prices at any point in the six months before departure, $1,016.

The old rule of thumb – booking tickets three months ahead for the lowest prices – seems now to be out of synch with the latest research.

The time of day you book tickets may make a difference, too. An on-going study, whose preliminary results were reported in the Economic Journal in 2010 by Marc Möller at the University of Bern and Makoto Watanabe at VU University in Amsterdam, suggests that airline tickets purchased around the world tend to be less expensive when booked in the afternoon, rather than in the morning. The authors considered the savings statistically significant, but didn’t reveal the exact amounts because research has yet to conclude. (Nighttime purchases, however, appear to offer no additional savings.)

One possible explanation is that airlines assume that people who book tickets in the morning tend to be business travellers who are willing to spend more money. But no one knows the reason for sure.  

The day of the week you book tickets may also make a difference., a website that has analysed a few billion fare searches since 2007, reports that Tuesday is typically the day when the most discounted fares are available for domestic US routes.

No one is certain why airlines tend to post the bulk of their fare sales at the start of the week. But some have speculated that it’s a habit held over from 15 years ago, when travel agents typically worked Monday-to-Friday schedules and airline-pricing analysts spent Mondays preparing to launch sales that appeared on Tuesdays — early enough for customers to visit the agents during the rest of the week. The pricing analysts may eventually lose this habit, though, as they adjust to the pace of always-open online travel agencies.

What’s true about timing in general may not be true for you specifically, so before booking a specific ticket, try the fare prediction tools at Bing Travel, a Microsoft-owned website that analyses millions of round-trip flight itineraries daily to determine pricing patterns. After you run a search for a US domestic or international trip (with at least one airport on the itinerary being in the US), Bing Travel makes a prediction about whether you should buy, because prices are predicted to go up later, or wait, because lower prices are expected. An independent audit of its predictions by Navigant, a consulting firm, found that the forecasts were correct approximately three out of four times.

As a downside, Bing’s price predictor only works on routes within the US or that include one US stop and are travelling to or from Canada, Europe or the Caribbean, all within a few months after purchase.

The bottom line
You have the best odds of finding the best deal on flights by making your purchase on Tuesdays in the afternoon, about three weeks in advance for domestic US tickets and a little more than a month in advance for international tickets.

Sean O’Neill is the travel tech columnist for BBC Travel