Final answer:
Characters in Geoffrey Chaucer's The Canterbury Tales gather for a pilgrimage to Canterbury, meeting at the Tabard Inn before setting off together.
Step-by-step explanation:
The event or circumstance that causes the characters to gather in Geoffrey Chaucer's The Canterbury Tales is a pilgrimage to the shrine of Saint Thomas Becket in Canterbury.
A group of pilgrims meets at the Tabard Inn in Southwark, near London, and they decide to travel together to Canterbury. Along the way, each pilgrim is invited to tell stories to entertain the group, which provides a rich tapestry of medieval life through the characters' diverse backgrounds and tales.