The Wikipedia description of the Birthday Problem:
In probability theory, the birthday problem or birthday paradox concerns the probability that, in a set of n randomly chosen people, some pair of them will have the same birthday. By the pigeonhole principle, the probability reaches 100% when the number of people reaches 367 (since there are 366 possible birthdays, including February 29). However, 99.9% probability is reached with just 70 people, and 50% probability with 23 people. These conclusions include the assumption that each day of the year (except February 29) is equally probable for a birthday.
Christmas time reminds me of this math oddity because of a couple I knew several years ago. They happen to have the same birthday, which is a bit unusual by itself. But that was only half of it. It turned out that their birthdays were both on December 25th, Christmas day.