Wordorigins.org

A busman’s holiday is a day off from work spent doing the things one does on the job. It comes from the idea that a bus driver would spend his holiday traveling somewhere on a bus. The phrase dates originally referred to horse-drawn buses. From the English Illustrated Magazine of 1893:

I shall indeed take a holiday soon,...but it will be a “Busman’s Holiday.”

(Source: Oxford English Dictionary, 2nd Edition)