What is It?

According to wikipedia

(therefore this MUST be true ;-))

slightly adapted though...

Polterabend is the term for a German wedding custom in which friends of the couple to be married break porcelain to bring luck to their marriage. The belief in the effectiveness is expressed by the old adage: "Shards bring luck". The Polterabend normally takes place in front of the house of the bride (or that of her parents), although exceptions are made for space considerations, for example. The couple generally announces the occasion but does not specifically send out individual invitations. Word spreads via word of mouth, and those with a desire to show up may do so. Many couples use this as a way of including people whom they are not able to invite to the wedding itself. Something to eat and/or drink is arranged. The actual high point of the custom is the throwing onto the ground of porcelain that has been brought. Forbidden are glasses (glass stands for happiness that should not be broken) and especially mirrors (as a broken mirror will bring seven years of bad luck). The couple must thereafter take care of cleaning up the pile of shards. This is supposed to make the couple aware that it will have to work together through difficult conditions and situations in life. It is possible that the Polterabend has a psychological motive: suitors who may have wished to have this bride for themselves have the opportunity to "let the steam out" in a socially appropriate manner, so that peace in the village could better be maintained.