What is the best time to buy an online airline ticket? The "6-week rule" (of thumb).

This is a question that keeps passengers and analysts/experts busy (https://fd.nl/werk-en-ge…/…/vliegen-het-kan-altijd-goedkoper), as much as the question how airlines price their seats considering the prices differences online customers encounter even on daily basis. The sometimes serious price differences even within the same day or between different people booking a rip at the same time have given food to urban legends and are often the source of customer frustration.
In 2011 the consumer advocacy program Tros Radar of the Dutch TV https://radar.avrotros.nl/uitzendingen/gemist/10-05-2011/vliegtickets/ asked me to help them find out if customer profiling by airlines using cookies was the reason for these differences. In the specific setting that we conducted the experiment at that time, we did not find any evidence of profiling. The conclusion was that the prices differences in online prices for the flights and the period we controlled were the result of capacity management algorithms. The findings were reported in my blog post of 11 March 2011 http://bit.ly/2HsAuOe
The question remains of course and is always timely: what is the cheapest time to book an online ticket? There are several studies in this, mostly by travel organizations or commercial parties. An older study (2012) http://business.time.com/…/study-the-best-time-to-buy-chea…/ and a more recent one (2017) https://nitravelnews.com/…/book-six-weeks-before-you-fly-f…/ agree that six weeks before departure is the best time to book an online ticket. Interestingly I did a small scale study covering a period of 2 months before departure for a return flight Amsterdam - Athens. I followed with Google Flights the prices of 3 airlines flying directly this route and the results were quite similar to the studies I mentioned: In this case, the lowest average price of the 3 companies (KLM, Aegean Airlines, and Transavia) was offered exactly 6 weeks before departure (42 days before departure). On the other hand, the cheapest price for Transavia over the whole period (119 Euro) and for Aegean Air (152 Euro) were noticed 60 days before departure while the cheapest price for KLM (170 Euro) was noticed for 4 times in a period between 40 and 45 days before departure. There are more interesting things to see in the ticket price development (see graphs) until the departure date.
It looks that the six-week rule holds some truth, Interestingly though, next to our 2011 study (that as a scientific article was accepted only by a conference) there is still no academic research on this issue. Maybe one of our next research projects for more scientific evidence on what is the best day to book a fight and how the algorithms of the airline capacity management work.