From Firing at Will, Chapter 6, "How to Fire an Employee":
Throughout the book, I talk about the need to fire a problem employee once you know that he or she is never going to work out. But there are exceptions, and Christmastime is one of them. Yes, I know it's the end of the year, and maybe there are tax or accounting implications. But don't do it. Either plan ahead and do it in November, or wait until a couple of weeks into the new year. Don't make the story "I got fired at the holidays."
One consideration that’s worth keeping in mind is the broader time frame. For example, it’s never a good idea to fire someone right before Christmas. That then becomes the story: “Those bastards fired me two days before Christmas.” That never sounds good, and it may actually increase the risk of a lawsuit.
Other days to avoid: The employee’s birthday. The day after her mother dies. The day after the Red Sox win the World Series. The basic rule here is this: be sensitive about the timing, but don’t worry about the “rules” about times and days.
Speaking of holidays, it's not too late to get a copy for your managerial loved ones' digital stockings. You can pick up the Kindle version here and the Nook version here. And if you want to go old school, the paperback version is available from Amazon here.