Happy Leap Second!
According to the (awesomely-named) International Earth Rotation Society, tomorrow night the minute 23:59 will be 61 seconds long, instead of the normal boring 60 seconds. This is called a leap second, and last occurred in 1998. Leap seconds are required to align the world's atomic and other super-accurate reference clocks with solar time, which varies slightly due to the earth's rotation not being perfectly constant. So instead of 23:59:59 followed by 00:00:00, tomorrow night (One Night Only!) the world's reference clocks will tick 23:59:59, 23:59:60, 00:00:00. Neat!
More details in the IERS's official bulletin. (via JWZ)