Flying overseas on Christmas eve or Christmas Day will most likely give you the cheapest price during that season.
Because almost everybody wants to be "home" with their family before that, the prices in the days before Christmas are much higher.
Also try to fly back as many days after New Year's as you possible can. The days right after New Year's have the higher prices as well and very busy.
For overseas travel however, flying back on January 1st sometimes is cheaper too.
Because not a lot of people want to be at the airport at 8 AM, when they partied all night on New Year's eve.
I've flown overseas during Christmas season in 2004 and 2005.
This year (2006) I traveled within the US on Christmas Day and was expecting it to be very quiet. But the airport was fogged in and so all the morning flights were delayed. So it was really crowded at the airport, because of that.