In the night of 1st. of July 2012 on 01:59:59 middle European time (UTC +2) there will be inserted a leap second to make the coordinated worldtime match the atomic time again. After 01:59:59 will come first 01:59:60 and then 02:00:00. The German atomic time transmitter DCF77 will announce the leap second before applying by setting the A2 bit in the time signal. This prevents the DCF77 clocks recognizing this leap second as a faulty transmission and ingnore it.
The default second has been defined in 1967 as 9.192.631.770 times the vibrating period of a cesium atom. This equals almost one full turn of the earth devided by 60 * 24 * 24. The rotation of the earth however is not constant and now the earth is turning a little slower then in 1967. So there is a time difference between the 'atomic time' and the 'world time UT1'. Therefore we use instead of UT1 the 'coordinated world time' UTC which kept the same as the atomic time by insering leap seconds. The last leap second was inserted 3.5 years ago.