Today is the Feast of Corpus Christi, which was inspired by a vision by the 13th-century canoness Juliana of Liège. The solemnity, celebrated on the first Thursday after Trinity Sunday in most of the world, but on the following Sunday in the U.S., is quite important to the history of medieval drama.
Juliana of Liège reported having a mystical vision of Christ in which she was told there must be a specific feast to celebrate the Lord’s true presence in the Eucharist. Juliana hesitated before describing the vision to her confessor, and perhaps with good reason. Her confessor spilled the beans to the bishop, who later proclaimed a Feast of Corpus Christi (Latin for "Body of Christ") in his diocese.
The feast remained a local affair until 1261. That’s when a former Archdeacon of Liège became Pope Urban IV. He waited until the last year of his papacy, however, 1264, to formerly institute the feast as part of the official calendar. He asked no less a personage than the great theologian Thomas Aquinas to write texts for a special mass. Still, celebrations of the feast spread slowly. How do you celebrate the theological doctrine that Christ is present in the host?
One idea was to take a consecrated host out of the church and parade it around the the town. This was also a time when plays were being performed outside, not just as liturgical events inside the church. The 12th-century Play of Adam, for instance, seems to have been performed in front of a church, given its stage directions. Why not process plays through the streets as well? Even after the tradition of parading through town with the host began to wane, a tradition continued of performing a procession of plays around the date of the feast.
It should be noted that Corpus Christi occurs in late May or early June, just as weather is getting really good in much of Europe, so it was an opportune time to perform outdoor theatre. The plays focused on how Christ saved humanity through the sacrifice of his body and blood, but Corpus Christi plays told the story of all creation, from the fall of the angels to the last judgement. In England, these plays were performed annually each spring, either on Corpus Christi or close to it, and came to be known as Mystery Plays.
In Christian theology, a religious truth known only through divine revelation is referred to as a Mystery. However, in the middle ages, the term "mystery" had another meaning as well. Each play in a cycle could be associated with a different guild or craft. The term "mysterium" denoted a craft, in part because a guild controlled the secret (or mystery) of how to perform a certain type of labor. Thus, during the middle ages, the term "mystery play" could have called to mind not just religious tenets, but also the practical skills possessed by the craftsmen who performed them.
If you're interested in reading a Mystery Play for Corpus Christi, a few years ago I modernized a portion of the play on the Scourging of Christ that was written by the famed Wakefield Master. The Wakefield Cycle is one of four cycles of English Mystery Plays that remain mostly intact, and many critics consider the plays attributed to the anonymous Wakefield Mast to be some of the best.