Click here to read about Leap Second Adjustment Day on Wikipedia.
An interesting chart of Leap Second Adjustments from the Wikipedia article:
Explanation of Leap Second Adjustment Day (which we are trying to understand):
Leap seconds are necessary partly because the length of the mean solar day is very slowly increasing, and partly because the SI second, when adopted, was already a little shorter than the current value of the second of mean solar time. Time is now measured using stable atomic clocks (TAI or International Atomic Time), whereas the rotation of Earth is much more variable.
