Leap Month
Amazingly, one of the best descriptions I've found of the Hebrew calendar comes from Microsoft's .NET Framework documentation. The month names are even given in Hebrew characters (and real characters, too, not images). If only anything else in that massive reference were anywhere near as useful.
You did know that the Hebrew calendar has a leap month 7 out of every 19 years, didn't you? Apparently, the length of the year was calculated with great precision using lunar eclipses, and the unusual calendar thus derived is extremely accurate. Or so says my uncle, though he also claims that I was born on February 29th. But better that than the first of אדר ב.