That poster makes me laugh every time I look at it. Wish I had one of my own. :-)
Here's what I found on the internets about the play:
History of the Play
The history of the Oberammergau Passion Play dates back to the middle of the Thirty Years War. In 1633, after months of suffering from the Bubonic Plague, the people of Oberammergau took a vow to perform the "Play of the Suffering, Dying and Resurrection of our Lord Jesus Christ" every ten years if they were spared. Miraculously from that point on they all survived and true to their promise, the first performance was in 1634. This simple performance was held in a meadow and acted by the villagers. After 1674 they decided to change the date, so as to fall every ten years beginning in 1680. The only time it was not performed, was during World War II. This time the play will open on May 15, 2010 and ends on October 3, 2010.
http://www.reformationtours.com/site/490868/page/927720