The Birthday Problem/Paradox
If you group 23 people born in the United States together, there is a 50% probability that two of them will have the same birth date (month and day).
If you increase the group to 47 people it becomes a virtual certainty that two of them will share a birth date.
There is a great deal of math that goes into this theory (also called the Birthday Paradox). Beyond the math, however, is the simple fact that the most common birthdays in the United States are in late September. (September 16 to October 1 are especially common.) It just so happens that these dates are 9 months after the holidays, Christmas in particular.
The point is that there are certain days on the calendar that are more likely to occur as birth dates than others. There are also certain forms of “randomness” created by computers that are far less random than you might expect – and their predictability makes cracking passwords easier as a result.