A bean-feast was primarily an annual dinner given by an employer to his or her employees. By extension, colloquially, it describes any festive occasion with a meal and an outing. The word, and its shorter form "beano," are fairly common in Britain, less known in the United States. In the nineteenth and early twentieth centuries, the beanfeast often took the form of a trip to some beauty spot, where the meal was provided. (e.g. ..I want a feast, I want a bean feast. Cream buns and doughnuts and fruitcake with no nuts, so good you could go nuts. - Veruca Salt, Willy Wonka & the Chocolate Factory)
According to the Encyclopedia Britannica Eleventh Edition, it is derivative of the feast of Twelfth Night at which a cake with a bean buried in it (a king cake) was a great feature. The bean-king was he who had the good fortune to have the slice of cake in which was the bean.