Seems eight weeks prior is the low-fare sweet spot in many instances, but not all:
When to buy airline tickets: The eight-week rule | The Economist
and
Why ?A = gUG + min(k-g, (1-g)(1-r)) equals low airline fares | World news | The Observer
I have personally flown long haul flight to Asia on Christmas day, and found that to be a good date to fly. Even got a complimentary upgrade one time.
There are other web sites that attempt to predict whether airfares will increase or decrease in the future.
The Microsoft search site, Bing, uses "farecast technoloyg" to predict trends:
Cheap flights, hotels, destinations - Bing Travel
There are others.
In general, I've found Tuesdays and Wednesdays have the lowest fares. This was true the other day when I booked a flight on DLH to BUD for October (about eight weeks in advance).