Best time to purchase air line ticket

It is simply. You buy the tickets, same as stock, when the price is the cheapest. The is no way to predict the prices. Their computer is smarter than yours. Set a reasonable price, if it hits - buy it, and never look back.

PS - Deleting cookies is an urban myth that has been debunked many times. Also review all of the postings on the same subject under Transportation in Trip and Tip Reports.

It is impossible to say what is the best time to buy an international airline ticket because prices move around a lot. However, there have been some nearly unbelievably low fares available this fall from some US origins to some European destinations. I don't know anything about Europe-to-US roundtrips but have a vague impression that they run lower than US-to-Europe roundtrips. I'm guessing there are some deals from Europe right now as well, though I don't know about the Dusseldorf-San Francisco market specifically.

Your daughter in law should be checking now in case something fabulous crops up. Not being a west coaster, I can't define "fabulous" for you, but perhaps your daughter-in-law has an idea of what a typical fare might be.

You have to first get a feel for what's "bad", "good" and "better" by tracking prices over time, preferably over a few months - only then you'll get a sense of when to bite. There is no one-size fits all "best time". Definitely play with dates, times, and even airports to see how it impacts prices...then you'll know what tradeoffs exist.

Yes - good suggestion from Agnes about "getting a feel." At that point, hit up Expedia or Travelocity or Orbitz (or all three!) and dial in a price tracker that will email you if the prices are going up - or down.

That way you don't have to keep checking in... an algorithm does it for you!

I also suggest configuring your alert settings on your phone so when that email comes in, you get an alarm. But then, I am VERY cheap, and didn't mind waking up a O-Dark-Thirty to score a deal. :-)

I assume you're talking about flying Air Berlin into Dusseldorf? But are you really going to Dusseldorf, or are you going into another city?
Air Berlin is a somewhat budget European air carrier--but not priced as low as RyanAir or EasyJet. They're making a bunch of operational changes trying to minimize operating losses approaching $500 million.
Who knows if they have the ability to cut airfares substantially at this time?
Another option would be to fly Norwegian Air Shuttle from Oakland into London-Gatwick, and picking up EasyJet for the rest of your flight.

This is FROM Dusseldorf to SFO. Mom must live in Germany.

Pricing for flights originating in Europe is different from US-based trips---often less expensive. So you may get lucky and find a great fare. I would start looking now. . .

As everyone else says, the only "tricks" are:

1) Look for prices now, to get a feel for what they are.
2) If you are locked into your arrival and departure cities and dates, don't worry about other fares. For instance, a sale on fares from London to New York won't help you - so don't try to compare it to what you have to pay. On the other hand, if you do have flexibility, do play around with all the options. For no clear reason, there can be significant differences.
3) Yes, fares originating in Europe can be completely different from fares originating in the US - for the same route (of course, as the airlines see it, it's not the same). So, don't even look at fares in the direction your passenger is not going - irrelevant for you.
4) Start by looking at Kayak or Google Flights, to see who flies your route with which connections. Then, look directly at the airline's websites, and sign up for alerts as said above.
4) Whenever you're looking at fares, have all the information necessary to book a ticket handy (so you can book right away and not miss a fare). When you see a low fare, book immediately; it can be gone in an hour. The airlines use computers, not humans, to adjust fares, so they go up and down quickly.
6) Once you've booked, don't keep looking, or you'll drive yourself crazy.

agree with previous posters. Airlines are one of the biggest purchasers of supercomputers and they use them to price fares to squeeze out every dollar they can. No rule of thumb is going to beat them.

The only "trick" I can suggest, given that your plans don't seem to involve a specific date and so is rather flexible, is to look at some of the mailing lists that reveal fare sales and be ready to jump on one you like as soon as it is posted. These sales are always a surprise, no rule of thumb predicts them. One archive is at http://archive.feedblitz.com/882882 and you can subscribe to get daily mails. I doubt you'll find many that list Düsseldorf so you may want to find out if there are any other airports she would be able/willing to fly out of. For example one recent post says

British Airways begins nonstop flights 4x weekly on 777-200 aircraft between Oakland, California OAK and London Gatwick LGW on March 28, 2017. To celebrate the launch, British Airways has introduced a special fare from Oakland to London for travel March 28, 2017 – May 11, 2017 and August 20, 2017 to October 28, 2017 from $599 in economy which includes all taxes, fees and charges. Premium economy begins from $1115. There are $694 round trip tickets available June to August.

Even with a nite in London on each side of the trip that may still be a roundtrip fare that is hard to beat.

How about Düsseldorf to LAX, then to SFO? Maybe more options? LTU did that route.

If it's from SFO to over there, say London, etc, get it now.

I would look at both Wow and Icelandic Air. You might find good prices. I use Icelandic Air to go from Frankfurt to Sacramento. The shorter trip from Iceland to Seattle is what attracts me as I don't want to spend 10-11 hours on a flight. Yes, it is 3 flights and travel time is a bit longer, but for me it is preferable.

Thank you for you effort to assist us! Her mom secured a round trip ticket--Ddort/SFO, with Air Berlin for 1300 Euros, 7/20 --8/16
We are all looking forward to her visit in Bay area

A few years ago we flew Air Berlin and were very pleased with them. Sounds like a nice visit. I'm glad it worked out for your family.