You can get this achievement anytime with a little help from console commands and your PC's date/time settings.
Let me start off by saying that in no way am I trying to take credit for this discovering this trick, I just figured I'd share it with everybody here.
First off, before you load the game, go in to your operating system’s date/time settings and change it to December 15, 2011; the time of day is irrelevant. Now we are ready for a couple of console commands.
To enable your L4D2 console, from the main menu go to:
>Options
>Keyboard/Mouse
>Allow Developer Console **Enabled**
From the main menu use the tilde (~) key to open the console. Once in the console type:
ss_map c1m1_hotel versus
This will start up the map Dead Center, once the map loads and you are in game, open the console again and type:
connect_splitscreen localhost 2
This will start split screen mode so that you don’t get kicked by the server for a lack of human players. If you are not a survivor, quit out and try again until you are. Play through the campaign looking for the green gift boxes that dropped when a special infected is killed. It makes no difference if you or the AI kills the specials as the drops are completely random. Once you see a gift simply run over it to pick it up. Repeat this process two more times and voila!
You may find it to be quicker after the first gift drop to quit out of that game, repeat the process above to start over. The first gift drops pretty quickly (within a few kills), but after that, it seemed to take a lot more kills to get another gift.
Now, why don’t you make like a tree... and get out of here?