Christmas Aliens should be watched as first episode of season 3.
I made this timeline years ago, if you're interested in this kind of stuff:
http://forums.thetechnodrome.com/showthread.php?t=34184
Quote:
Originally Posted by TheSkeletonMan939
Someone here once said that the biggest indication that the episode is "supposed" to take place at the very beginning of the season is that Leatherhead isn't at their Christmas party.
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That's one reason. Plus the PD have regular guns. After the invasion, there was a lot of alien tech being used by the gangs. Also production code.