Ever read an article about the best time to book a flight? Travel “gurus” often mention Thursday. “Midnight” is also popular. Or you’ll often see “right now,” making sure to pique your interest before then going on to give advice such as Close to departure, prices are more likely to go up (while others say down) … and … You want to start looking for deals about six months before travel … and … In general the cheapest tickets are available two to three months before travel … and … But for peak holiday travel, the cheapest tickets are available earlier than that … and … You don’t want to buy 11 months or a year in advance.

Perhaps it’s the Ides of March or when the sun’s shadow is at 37 degrees at Stonehenge. Yes, it can be rather arbitrary.

But I think of the best time to book a flight differently.

The FCF Mindset

First of all you have to remember that many of these articles are based on economy travel—which is very different from premium travel—and they probably cover every or many regions of the world. So Europe is lumped into the same box as Asia and the Pacific. But different regions have different fare restrictions and airlines price regions differently.

First Best Time to Book: When Fares Are Ridiculously Low

One thing is true across the board for all regions and classes: Lucky Fare Deals, or as we call them at FCF, “Flash EasyUp Fares.” These come and go within hours or days, can be restricted to certain departure and destination cities, certain airlines, may only be valid for travel during a certain season, and can vary how far in advance fares have to be booked.

Recent Lucky Fare deals FCF spotted for Business Class included: Italy for $1,757, to Nairobi for $2,146, to Hawaii for $897, to Mumbai for $2,382, to Tokyo for $1,997, to Frankfurt for $1,996, to Seoul for $1,997, and to Manchester for $1,996.

FCF’s team watches for these fares like a scientist looks into a microscope. But you’ve got to be constantly looking into the microscope, day in, day out.

Next Best Time(s) to Book:

What do you do when you don’t have a Lucky Fare Deal to Europe? Here are six pieces of advice to keep in mind. Also remember, this is a general guideline; if you wait too long, flights can fill up and the low-fare buckets disappear, so all fares citied are based on inventory availability.

Know About the Five-Month Trend

Most often—about 90% of the time—the lowest Business Class fares to Europe require a 150-day advance purchase to get the best deal. If you haven’t followed FCF’s reporting long and are hearing this for the first time, don’t feel bad because this trend is only about eighteen months old.

Know You Can Buy 100 Days for $500

Miss the 150-day advance fare, and you can often end up paying about $1,000+ more. However, some destinations/routes in Europe sometimes offer 50-day advance purchase fares for only $500 more than the lowest fare. So check these cities when you are still more than 50 days from departure. For example, Dallas-Frankfurt on American goes for $3,618 with a 150-day advance purchase mid-week, miss it, and the fare increases by $500 to $4,118.

Know About Pressure-Relieving, 28-Day Advance-Purchase Destinations

These routes often offer their lowest fares with only a 28-day advance purchase requirement. For example: New York-Madrid on SkyTeam and oneworld starts at $2,500 (New York-Paris can be as much as $7,118), Los Angeles-Copenhagen at $3,400, and Chicago-Madrid at $3,300. Most U.S. carriers have at least one European destination with a 28-day purchase fare. That can be the difference between paying $2,500 and $8,000.

Know About the “I Want to Leave Today” Route(s)

On SkyTeam airlines Air France, Alitalia, Delta, and KLM, the lowest Business Class fare on the New York-Frankfurt route starts at $2,494 and does not require an advance purchase.

Know About the Lesser-Known Airlines

Scandinavian Airlines, a Star Alliance (United) partner, offers new lie-flat Business Class seats, flies from Boston, Chicago, Los Angeles, Miami, New York, San Francisco and Washington, DC to about 100 cities in Europe, offering connections in its hub cities, Copenhagen and Stockholm, but requires only a 28-day advance purchase for its lowest Business Class fares on most routes. Other airlines that offer travelers such freedom are airberlin (16-day advance purchase on many fares) and LOT Polish (3- to 28-day advance purchase on many fares).

Know How to Decode Advance Purchase Requirements

Generally speaking, my favorite search tool is Google Flights. Use it early in your planning process to get a general lay of the When-to-Book Land. Just input your route and class of service, filter by stops and preferred airline/alliance, then click on the departure date, and a fare calendar comes up. It’s very user-friendly. You can survey months of fares before and ahead of your planned departure date. This will give you an idea of how far in advance—or when—the lowest fares are available.

Of course, not everybody is so well organized, or wants to be. At FCF, we often talk of the benefits of being a bags-packed traveler to take advantage of short-term travel opportunities. Here it’s more about long-term thinking. Either way, FCF just wants to start you off with eyes wide open.