Every year since 1977 the clocks are adjusted twice each year. In spring the clocks are turned one hour forward and in fall they are turned back again. To distinguish between the regular CET and the daylight savings time, this is called CEST.
While the clock changing happened in April and September at first, the countries negotiated to change the time coordinated during the nights at 2 a.m. at the last Sunday in March and in October. Thus, this night will be the shortest, because at 1 a.m. the clocks will be set to 2 a.m.
The reason for this is to save energy by prolonging the hours with available daylight.
I personally don’t like it, because of the time changing I have to stand up one hour earlier and thus being more tired then usual for some week until my body will have catched up.
On the other hand, this does not save energy or money, because people use the brighter evening hours for several activities and using additional energy for that i.e for driving by car to leisure or sports parks. Also, insurance companies have reported increased amounts of accidents during the 4 weeks after the time changing. Tiredness is also a problem and in factories.