It was 1895, and in the studio of Alphonse Mucha. That's all the context that we have. We don't know why Post-Impressionist painter Paul Gauguin took off his pants. But we should not be surprised. William-Adolphe Bouguereau would live for another ten years, but the Academic traditions of realistic representation and wearing pants were already dead.
Link -via Retronaut
http://www.youtube.com/watch?v=ZLE60Aspsu4
Which is NSFW, by the way.