I don't know about the rest of the country, but here in the NorthEast President's Week is also many states' winter vacation. (we get a week at Christmas, a week in Feb, and a week in April). So it's a MUST buy early for me, if we want to get affordable flights for vacation. The outbound flights are generally very reasonable, coming home is usually much more expensive.
I booked our outbound flight (BOS to MCO) on February 12, 2008 at midnight on Saturday the 17th. $99 for the one way. I've paid less in the past, and I've paid MUCH MUCH more. I'm looking to fly home either the 22nd or 23rd, so I have about another week before I start looking at those flights.
Now, for the same flight I booked, but flying a day later, is $269 for the one way.
This is on Delta.