That's true for most games, especially console titles where purchasing is often done by parents on behalf of their kids. For something like Half Life 2, the ideal release date is early-mid november. This way, they can give review code to all the major magazines and have the game reviewed for the november issue, just before the game is on the shelves. If they can get massive coverage in all the top magazines a week or two before release interest will be at the maximum and sales will rocket. Also, all the major sales outlets will have just been paid to set up HL2 displays in their stores until mid january, maximising post xmas sales. Also, with november, all the serious gamers purchase on day of release then start spreading the word. By christmas, every gamer with friends wants HL2 and has just got a load of money for christmas - cha-ching!
Remember that word of mouth sales will continue on after xmas and will be the main sales promotion for the few months after xmas - lots of people got HL2 for xmas and so are now telling their friends how great it is and are promoting the game for free. Adults are usually skint after xmas, but kids are wealthy and want to spend their xmas earnings.
Look at any site that has a list of game release dates, the hotspots are mid october to mid november. December is a very dry month for actual releases - only games that are forced to slip back are released during december in a desperate bid to get some of that xmas booty.