When I picture dinner in medieval England or France for example, I picture a big feast with a table laden with all sorts of food and everybody just digs in and eats until they can't eat anymore.
In the modern western world (i.e. Europe, North America, Australia, New Zealand etc.) when we go out for dinner we usually eat several courses. We get a portion of the meal as a starter, as a main course and as a dessert, which are usually combined so the flavours compliment each other. There is also matching etiquette during dinner that seems absent during the medieval period.
How and why did this change occur? Did it happen gradually or was there a culinary revolution?
Eating all at once (service à la Française) was the norm in France in the early-mid 19th century. Chef Marie-Antoine Carême, thought of as the first celebrity chef, brought the "eating in an order chosen by the chef" (service à la Russe) style of service to the French royalty, where trends go to spread to the rest of the world.
Chef Auguste Escoffier, following in Carême's footsteps, spread Russian service, as well as updating other developments in French cuisine through to the early 20th century.
Together, these two chefs are known as the two that established modern French cuisine and the profession of cooking as legitimate.
Source: Accounting for Taste, the Triumph of French Cuisine and wikipedia articles on Carême and Escoffier