Flight tickets: buy two weeks before even during holiday seasons?

There doesn't seem to be a clear pattern - I think it depends on the demand for the tickets for a particular day. My best guess is that they manage to sell more tickets than expected (at a given time to departure) than the price goes up, and if less than it goes down.

In general I think the cheapest way is to find some promotion for the tickets (you need luck here I guess). Then I heard from someone that the optimal buying time before flight is 6-8 weeks, but again I haven't seen any evidence.

In my experience with flights within Asia, and also trans Atlantic and Pacific flights from North America, yes, flights are cheaper 2 weeks or more before your date. However, it is helpful to search for flights and prices for the route you are interested in way before you plan to buy. The reason for this is to get an idea about what a "base" price will be against which you plot the price you are willing to pay, rather than what the cheapest fare the airline/search site offers. So, from my city to Chiang Mai Thailand, I discovered that on average 650 dollars would get me to either Shezhen, Guangzhou, or Shanghai, from which I could get flights to Bangkok/Chiang Mai. I also discovered flights to Hong Kong were actually 350 dollars. But for some reason at that time flights between HK and Thailand were more expensive than flights departing from within China. So I resigned myself to approximately 700-1000 dollars to get to Chiang Mai. Various other sites, routes, and airlines were all within that window, and that price was for departures 2 weeks from any given day I was searching. Flights even a day sooner, or later, than two weeks from the date I was searching, pushed the base price to 850 dollars.

Then all of a sudden a China Eastern flight popped up for the dates I wanted to fly, but 40 days before I would fly, and the fare was 440 dollars, all the way to Chiang Mai (on chipmunk.com). I didn't hesitate to buy that. Maybe it would be cheaper, BUT, compared against the base rate I was finding, it was substantially lower, now. So I bought it, and in this case, that price lasted about a week and then shot up like all the rest. I know it wasn't the cheapest ever, but I consider this to mean I saved 400-600 dollars, based on knowing on average what most flights were costing.

Try many different airlines, search sites, origin of flight, etc.

A few major points: the person who did this neat little comparison - nice job by the way - looked on all different days of the week.

Oct 23rd and Nov. 7th were Wednesdays, Nov. 24th was a Thursday, and Dec. 16th was a Friday.

It's well-known that tickets prices change, often dramatically, during the week. They are generally lowest Tues (sometimes also Mon and Wed) and gradually rise to highs on Fri and Sat. It would be more efficient to compare the price always on the same day - so always on a Tuesday but different weeks, for example.

Secondly, it makes little sense to say "the Dec. 23rd ticket might have cost $229 if I have booked it Oct. 23rd" based on the reasoning that "now it's Nov. 23rd and the price of a Jan 23rd ticket is $229." A Dec. 23rd ticket will likely to be higher than a Jan. 23rd ticket no matter WHEN you book it. Even if you book it in January of the previous year, it's just more expensive to travel around a holiday because there is more demand at that time (i.e. airlines can get more money for those tickets). It's not just about the span of time between when you book and when you fly. Actually, one of the best tips for securing a plane ticket at a low price is to AVOID the holidays - problem is, that's when most of us want to (or are able to) travel!

Thus, the supply-and-demand nature of this aspect of flying holds more control over the price than the period of time between when you book and when you fly.

I think there isn't a general answer. Generally, the prices raise slowly and then they drop shortly before the departure, but only if the plane doesn't get full.

So it is more or less a game. If you think the tickets won't be sold out I would wait for the very last moment. Otherwise I would save the ticket as soon as possible.

Since you're traveling in the holiday season I would rather chose to be on the safe side and book the ticket as early as possible.

ticket prices change wildly in the period prior to a flight. They can go up, but also down. Usually they are very several months before flight (most airlines open bookings 6 months to a year before flight departure), go down to a bottom maybe a few weeks early, up again to a day or two before departure, then down again as the airline scrambles to fill the last few seats.
It's all a combination of risk, convenience, and opportunity. If you can afford to miss a flight, you could wait until maybe a day before departure and try to get a dirt cheap ticket. If you MUST catch that flight, you might want to book half a year in advance.

As cheaper booking codes / fare classes are sold out the remaining seats typically have higher prices. With planes flying so full lately (especially during the holiday seasons), it tends to be a good idea to book early. When looking for a good fare one can try to determine what the average fare is (e.g. at sites like flightrouting.com) to be able to tell if the fares that are available are better than what's typical for that route. In the holidays it seems that it would be best to book earlier than just two weeks, especially as some flights may completely sell out.