If you go on A Busman's Holiday you spend your time away from work doing exactly what you'd during your working day. The idiom comes from the beginning of the 20th century, when buses were horse-drawn. By tradition, a bus driver would spend his days off travelling in the rear of his bus in order to make certain that the relief driver was looking after his horses properly. The horse was the means of his income and so the animal's welfare was essential and drivers took few chances.