Question: In what century was the sign of peace (the kiss of peace) officially installed into the rite of the Mass? Why is it fitting for the sign of peace to be before Communion?
Answer: The sign of peace (kiss of peace) was a part of Mass as early as the second century. The laity were excluded from sign of peace during the Middle Ages and the practice was eventually dropped. In the 20th century, Vatican II restored the practice to all participants at mass. It is fitting for the sign of peace to be before Communion because it is a witness to the Christian belief that true peace is a gift of Christ's death and resurrection.
For More Reading
http://www.catholicculture.org/culture/library/dictionary/index.cfm?id=34444
http://www.cptryon.org/ask/ask/signpeace.html
http://www.catholicnews.com/data/stories/cns/1403223.htm