Closing out the year
The effect is exacerbated in many organisations which run their leave year from January to December, with a "use it or lose it" policy. So many people finished working a week or even a fortnight before Christmas. Which is nice for the people involved, but it does cause problems for the rest of us.
For instance, my final project of the year was a data cleansing exercise, applied in-flight to a production database. The cleansing was done in overnight batches, which we estimated would take about two weeks to run. The initial deployment was a time consuming exercise, which required a large chunk of downtime and also applied a massive change to the database. If it went wrong, we would have to rollback everything and start all over again. So that meant we could only deploy at a weekend. As if that wasn't exciting enough, the exercise absolutely had to be completed by 31-DEC-2011.
The advent season added further spice. Office parties, nativity plays, Christmas shopping and using up leave all made it harder to arrange meetings. We couldn't rely on finding the necessary people to sign off documents, answer questions, provide technical support or undertake UAT. Plus of course the entire development team wanted to celebrate Winterval. If we didn't have made a successful deployment early in December we wouldn't be able to get the cleansing finished before Christmas. That would have thrown we did a lot of people's Christmas plans into chaos.
Fortunately we deployed cleanly first time. And the cleansing proceeded smoothly. In fact we made it with, oh, days to spare. So that was a nice way to round off the year, but it felt pretty hairy at the the time.
Next year I will be grateful if the Grinch isn't in charge of project scheduling.