Is calling an office Xmas party a “bounenkai” (forget the year party) just a joke, or does it have a more traditional and/ or deeper meaning?

Although most people can see the humourous connection being drinking and forgetting, the original meaning seems to have been to have a drink with someone to forget any disagreements or problems you might have had over the year so that you can start with a blank slate next year.

More on Japanese traditional festivals here.