What event or circumstance causes the characters to gather?
The event or circumstance that causes the travellers to gather in The Canterbury Tales by Geoffrey Chaucer is the concept of 'pilgrimage.' In Chaucer's day, an annual holiday was not a tradition for ordinary people - any 'rest' the labourers or peasants got was tied in to religious feasts or saints days and connected to religious observance and church services. Wealthier people fared better however, but even their 'breaks' were ruled by religious observance. Prosperous citizens felt obliged to give thanks for their good fortune, and to share a little of it in alms giving - and also to 'make up' for their easier lives by doing penance in a different way. One of the ways to achieve this (and to achieve a few 'brownie points' or credits in heaven) was to take on a long pilgrimage to honour a saint. The handy part of this arrangement was that a pilgrimage,by nature of the slow form of transport by donkey or pony, inevitably became a holiday of sorts from one's usual occupation or work.
An interesting angle on the 'holiday' notion of pilgrimage used by Geoffrey Chaucer in the Tales is the idea that a large variety of people from varying occupations, classes and regions could all be mixed up together on these religious journeys or 'holy days.' There are numerous story tellers of differing status, class and personality, appearing in many different forms in Chaucer's tales but they all have one thing in common - each character is a pilgrim. In The Canterbury Tales the author shows the diversity of his contemporary social situation and also paints a picture of the relationships, conflicts and hidden rivalries between different merchants, traders and professions. The context of 'pilgrimage' was an ideal platform for depicting the entertainment angle too - the songs, music and stories the characters relied upon to while away the time until they arrived at their destination.